Add some basic git command argument prediction

[daxian-dbw]

PR Summary

Add argument prediction for some basic git command: git merge, git branch -D, git checkout, git checkout -b, git push.

I use those commands frequently in my daily work, targeting different branches and remotes, from different repos. The history predictions in this case can be very misleading and sometimes annoying, because the branch/remote names in the history prediction are often incorrect.

I have been wanting to enhance the CompletonPredictor to handle those git scenarios, and this is the first step towards that.
It's impossible to run git to get the branch/remote data for a predictor due to the timeout restriction, so I choose to access the files/folders under .git directly to get the branch/remote information about a repo.