add a setting to set git binary path

[MasseGuillaume]

git has a new vulnerability CVE-2025-48384 that is patched in 2.50.1
however, the default version for git on apple via xcode dev tools is 2.39.5

/usr/bin/git --version
git version 2.39.5 (Apple Git-154) 

and it's not possible to remove/symlink /usr/bin/git even with sudo.

I would like to be able to configure it's path in zed:

zed/crates/zed/src/main.rs

Line 293 in d1a6c5d

let git_binary_path =