Exit early from git plugin if not in git repo.

[jediant]

Added check for .git directory in current, before wasting time querying git. This saves seconds on my pc.

[robbyrussell]

@jediant It looks like causes some quirks where current_branch stops working when you're in a subdirectory. I need to revert this for now. Can you think of a better solution that meets both scenarios?

[ncanceill]

Yeah, as @blueyed had pointed out prior to the time of merging this, this fix does not work at all.

git commands exit pretty quickly when not in a repository anyway, so this was removed from #2790.

[gsemet]

Please revert this patch. This is invalid since git command work in subdirectory. You need to look for every parent of the current directory to see if these isn't a .git directory

[ncanceill]

Yes, #2790 reverts this.