That error message can happen because of operations on a missing or moved file:
See issue 212 (for ST2, but possibly relevant for ST3).
Ok, I think why this bug happened.
There was a file that was moved to another location form outside ST2, and Git plugin failed to do something to the file because it no longer existed, but that situation was interpreted as missing git command.
Another workaround mentions:
Installing python via homebrew and restarting ST2 fixed this problem for me:
brew install python
Also, by David Gee -- davidagee :
On OS X Mountain Lion, the only way I was able to fix this, after trying everything in this thread, and additional mucking around with path settings, was to open "/Users/[me]/Library/Application Support/Sublime Text 2/Packages/Git/Git.sublime-settings" and to change git_command to "/usr/local/git/bin/git".
Adding a git_command setting to user prefs doesn't work.
The plugin seems to ignore the value when set in Preferences.sublime-settings.