Why can't I use TortoiseMerge as my git merge tool on Windows?

If you have the latest git, run this command line once:

git config merge.tool tortoisemerge

Important: Do not add a .exe extension to the command.

If that fails, or if you want to add a different merge tool that git doesn't know about, do this:

Open one of the following in an editor:

• 64-bit git: C:\Program Files\Git\mingw64\share\git-gui\lib\mergetool.tcl
• 32-bit git: C:\Program Files (x86)\Git\share\git-gui\lib\mergetool.tcl

Add something like this to mergetool.tcl:

tortoisemerge {
    set cmdline [list TortoiseMerge.exe -base:"$BASE" -mine:"$LOCAL" -theirs:"$REMOTE" -merged:"$MERGED"]
}

Put the new tortoisemerge entry above this other code:

default {
        error_popup [mc "Unsupported merge tool '%s'" $tool]
        return
}

Bonus example:

To use SourceGear diffmerge, add this to mergetool.tcl:

diffmerge {
    set cmdline [list "C:/Program Files (x86)/SourceGear/DiffMerge/DiffMerge.exe" --merge --result=$MERGED $LOCAL $BASE $REMOTE]
}

Try this:

[merge]
tool = tortoise

[mergetool "tortoise"]
cmd = "TortoiseMerge.exe" -base:"$BASE" -theirs:"$REMOTE" -mine:"$LOCAL" -merged:"$MERGED"

Source: http://programmersunlimited.wordpress.com/2010/07/01/getting-git-to-use-tortoisemerge/

For the case that I was working on, the mergetool was already set to tortoisemerge, but it could not find it.

Providing a fully qualified location worked in Windows:

git config --global mergetool.tortoisemerge.cmd "\"C:\\Program Files\\TortoiseGit\\bin\\TortoiseGitMerge.exe\" -base:\"$BASE\" -mine:\"$LOCAL\" -theirs:\"$REMOTE\" -merged:\"$MERGED\""

Try typing TortoiseMerge.exe from the command line to make sure it's in the path. If not add it via My Computer > Properties > Advanced > Environment Variables > System Variables:Path.

Then from command prompt configure it via the following commands

git config --replace --global diff.tool tortoisemerge
git config --replace --global difftool.diffmerge.cmd "TortoiseMerge.exe -base:\"$BASE\" -theirs:\"$REMOTE\" -mine:\"$LOCAL\" -merged:\"$MERGED\""
git config --replace --global difftool.prompt false

To use it from the command prompt, type git difftool from your within your git working directory.

It shows the files one at a time so you're still better off installing TortoiseGit which makes things easier to handle, even if just for the diff part.

This issue appears resolved in the latest git (I have git version 1.9.4.msysgit.1).

C:\git\build>git mergetool

This message is displayed because 'merge.tool' is not configured.
See 'git mergetool --tool-help' or 'git help config' for more details.
'git mergetool' will now attempt to use one of the following tools:
tortoisemerge emerge vimdiff

C:\git\build>git config merge.tool tortoisemerge

C:\git\build>git mergetool
No files need merging