git pull while not in a git directory

Starting git 1.8.5 (Q4 2013), you will be able to "use a Git command, but without having to change directories".

Just like "make -C <directory>", "git -C <directory> ..." tells Git to go there before doing anything else.

See commit 44e1e4 by Nazri Ramliy:

It takes more keypresses to invoke Git command in a different directory without leaving the current directory:

1. (cd ~/foo && git status)
git --git-dir=~/foo/.git --work-tree=~/foo status
GIT_DIR=~/foo/.git GIT_WORK_TREE=~/foo git status
2. (cd ../..; git grep foo)
3. for d in d1 d2 d3; do (cd $d && git svn rebase); done

The methods shown above are acceptable for scripting but are too cumbersome for quick command line invocations.

With this new option, the above can be done with fewer keystrokes:

1. git -C ~/foo status
2. git -C ../.. grep foo
3. for d in d1 d2 d3; do git -C $d svn rebase; done

Since Git 2.3.4 (March 2015), and commit 6a536e2 by Karthik Nayak (KarthikNayak), git will treat "git -C '<path>'" as a no-op when <path> is empty.

'git -C ""' unhelpfully dies with error "Cannot change to ''", whereas the shell treats cd ""' as a no-op.
Taking the shell's behavior as a precedent, teach git to treat -C ""' as a no-op, as well.

4 years later, Git 2.23 (Q3 2019) documents that 'git -C ""' works and doesn't change directory

It's been behaving so since 6a536e2 (git: treat "git -C '<path>'" as a no-op when <path> is empty, 2015-03-06, Git v2.3.4).

That means the documentation now (finally) includes:

If '<path>' is present but empty, e.g. -C "", then the current working directory is left unchanged.

You can see git -C used with Git 2.26 (Q1 2020), as an example.

See commit b441717, commit 9291e63, commit 5236fce, commit 10812c2, commit 62d58cd, commit b87b02c, commit 9b92070, commit 3595d10, commit f511bc0, commit f6041ab, commit f46c243, commit 99c049b, commit 3738439, commit 7717242, commit b8afb90 (20 Dec 2019) by Denton Liu (Denton-L).
^{(Merged by Junio C Hamano -- gitster -- in commit 381e8e9, 05 Feb 2020)}

t1507: inline full_name()

^{Signed-off-by: Denton Liu}

Before, we were running test_must_fail full_name. However, test_must_fail should only be used on git commands.
Inline full_name() so that we can use test_must_fail on the git command directly.

When full_name() was introduced in 28fb84382b ("Introduce <branch>@{upstream} notation", 2009-09-10, Git v1.7.0-rc0 -- merge), the git -C option wasn't available yet (since it was introduced in 44e1e4d67d ("git: run in a directory given with -C option", 2013-09-09, Git v1.8.5-rc0 -- merge listed in batch #5)).
As a result, the helper function removed the need to manually cd each time. However, since git -C is available now, we can just use that instead and inline full_name().

Edit:

There's either a bug with git pull, or you can't do what you're trying to do with that command. You can however, do it with fetch and merge:

cd /X
git --git-dir=/X/Y/.git fetch
git --git-dir=/X/Y/.git --work-tree=/X/Y merge origin/master

Original answer:

Assuming you're running bash or similar, you can do (cd /X/Y; git pull).

The git man page specifies some variables (see "The git Repository") that seem like they should help, but I can't make them work right (with my repository in /tmp/ggg2):

GIT_WORK_TREE=/tmp/ggg2 GIT_DIR=/tmp/ggg2/.git git pull
fatal: /usr/lib/git-core/git-pull cannot be used without a working tree.

Running the command below while my cwd is /tmp updates that repo, but the updated file appears in /tmp instead of the working tree /tmp/ggg2:

GIT_DIR=/tmp/ggg2/.git git pull

See also this answer to a similar question, which demonstrates the --git-dir and --work-tree flags.

You may wrap it in a bash script or git alias:

cd /X/Y && git pull && cd -