How do I tell git to always select my local version for conflicted merges on a specific file?

On the specific instance of a config file, I would agree with Ron's answer:
a config should be "private" to your workspace (hence "ignored", as in "declared in a .gitignore file").
You may have a config file template with tokenized values in it, and a script transforming that config.template file into a private (and ignored) config file.

However, that specific remark does not answer what is a broader more general question, i.e. your question(!):

How do I tell git to always select my local version for conflicted merges on a specific file ? (for any file or group of file)

This kind of merge is a "copy merge", in which you will always copy 'ours' or 'theirs' version of a file whenever there is a conflict.

(as Brian Vandenberg notes in the comments, 'ours' and 'theirs' are here used for a merge.
They are reversed for a rebase: see "Why is the meaning of “ours” and “theirs” reversed with git-svn", which uses a rebase, "git rebase, keeping track of 'local' and 'remote'")

For "a file" (a file in general, not speaking of a "config" file, since it is a bad example), you would achieve that with a custom script called through merges.
Git will call that script because you will have define a gitattributes value, which defines a custom merge driver.

The "custom merge driver" is, in this case, a very simple script which basically will keep unchanged the current version, hence allowing you to always select your local version.

IE., As noted by Ciro Santilli:

echo 'path/to/file merge=ours' >> .gitattributes
git config --global merge.ours.driver true

Let's test that in a simple scenario, with a msysgit 1.6.3 on Windows, in a mere DOS session:

cd f:\prog\git\test
mkdir copyMerge\dirWithConflicts
mkdir copyMerge\dirWithCopyMerge
cd copyMerge
git init
Initialized empty Git repository in F:/prog/git/test/copyMerge/.git/

Now, let's make two files, which will both have conflicts, but which will be merged differently.

echo a > dirWithConflicts\a.txt
echo b > dirWithCopyMerge\b.txt
git add -A
git commit -m "first commit with 2 directories and 2 files"
[master (root-commit) 0adaf8e] first commit with 2 directories and 2 files

We will introduce a "conflict" in the content of both those files in two different git branches:

git checkout -b myBranch
Switched to a new branch 'myBranch'
echo myLineForA >> dirWithConflicts\a.txt
echo myLineForB >> dirWithCopyMerge\b.txt
git add -A
git commit -m "add modification in myBranch"
[myBranch 97eac61] add modification in myBranch

git checkout master
Switched to branch 'master'
git checkout -b hisBranch
Switched to a new branch 'hisBranch'
echo hisLineForA >> dirWithConflicts\a.txt
echo hisLineForB >> dirWithCopyMerge\b.txt
git add -A
git commit -m "add modification in hisBranch"
[hisBranch 658c31c] add modification in hisBranch

Now, let's try to merge "hisBranch" upon "myBranch", with:

• manual resolution for conflicting merges
• except for dirWithCopyMerge\b.txt where I always want to keep my version of b.txt.

Since the merge occurs in 'MyBranch', we will switch back to it, and add the 'gitattributes' directives which will customize the merge behavior.

git checkout myBranch
Switched to branch 'myBranch'
echo b.txt merge=keepMine > dirWithCopyMerge\.gitattributes
git config merge.keepMine.name "always keep mine during merge"
git config merge.keepMine.driver "keepMine.sh %O %A %B"
git add -A
git commit -m "prepare myBranch with .gitattributes merge strategy"
[myBranch ec202aa] prepare myBranch with .gitattributes merge strategy

We have a .gitattributes file defined in the dirWithCopyMerge directory (defined only in the branch where the merge will occurs: myBranch), and we have a .git\config file which now contains a merge driver.

[merge "keepMine"]
        name = always keep mine during merge
        driver = keepMine.sh %O %A %B

If you do not yet define keepMine.sh, and launch the merge anyway, here is what you get.

git merge hisBranch
sh: keepMine.sh: command not found
fatal: Failed to execute internal merge
git st
# On branch myBranch
# Changed but not updated:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#
#       modified:   dirWithConflicts/a.txt
#
no changes added to commit (use "git add" and/or "git commit -a")

type dirWithConflicts\a.txt
a
<<<<<<< HEAD:dirWithConflicts/a.txt
myLineForA
=======
hisLineForA
>>>>>>> hisBranch:dirWithConflicts/a.txt

That is fine:

• a.txt is ready to be merged and has conflict in it
• b.txt is still untouched, since the merge driver is supposed to take care of it (due to the directive in the .gitattributes file in its directory).

Define a keepMine.sh anywhere in your %PATH% (or $PATH for our Unix friend. I do both of course: I have an Ubuntu session in a VirtualBox session)

As commented by lrkwz, and described in the "Merge Strategies" section of Customizing Git - Git Attributes, you can replace the shell script with the shell command true.

git config merge.keepMine.driver true

But in the general case, you can define a script file:

keepMine.sh

# I want to keep MY version when there is a conflict
# Nothing to do: %A (the second parameter) already contains my version
# Just indicate the merge has been successfully "resolved" with the exit status
exit 0

(that was one simple merge driver ;) (Even simpler in that case, use true)
(If you wanted to keep the other version, just add before the exit 0 line:
cp -f $3 $2.
That's it. You merge driver would aways keep the version coming from the other branch, overriding any local change)

Now, let's retry the merge from the beginning:

git reset --hard
HEAD is now at ec202aa prepare myBranch with .gitattributes merge strategy

git merge hisBranch
Auto-merging dirWithConflicts/a.txt
CONFLICT (content): Merge conflict in dirWithConflicts/a.txt
Auto-merging dirWithCopyMerge/b.txt
Automatic merge failed; fix conflicts and then commit the result.

The merge fails... only for a.txt.
Edit a.txt and leave the line from 'hisBranch', then:

git add -A
git commit -m "resolve a.txt by accepting hisBranch version"
[myBranch 77bc81f] resolve a.txt by accepting hisBranch version

Let's check that b.txt has been preserved during this merge

type dirWithCopyMerge\b.txt
b
myLineForB

The last commit does represent the full merge:

git show -v 77bc81f5e
commit 77bc81f5ed585f90fc1ca5e2e1ddef24a6913a1d
Merge: ec202aa 658c31c
git merge hisBranch
Already up-to-date.

(The line beginning with Merge does prove that)

Consider you can define, combine and/or overwrite merge driver, as Git will:

• examine <dir>/.gitattributes (which is in the same directory as the path in question): will prevail upon the other .gitattributes in directories
• Then it examines .gitattributes (which is in the parent directory), will only set directives if not already set
• Finally it examines $GIT_DIR/info/attributes. This file is used to override the in-tree settings. It will overwrite <dir>/.gitattributes directives.

By "combining", I mean "aggregate" multiple merge driver.
Nick Green tries, in the comments, to actually combine merge drivers: see "Merge pom's via python git driver".
However, as mentioned in his other question, it only works in case of conflicts (concurrent modification in both branches).

As @ciro-santilli has commented, the simple way to do it to use .gitattributes with settings it:

path/to/file merge=ours

and enable this strategy with:

git config --global merge.ours.driver true

(I am adding this as an answer to make it more visible but making it a Community Wiki to not try to get above user's credits for myself. Please upvote his comment under the Q here to give him kudos!)