Adds git config vars to customize behavior of git bash completion

[sagemintblue]

Due to the high volume of changes in Twitter's source monorepo, the existing behavior of git bash completion is bad: By default, completion of git ref names includes patterns refs/heads, refs/tags, and refs/remotes. As of this writing, patterns refs/tags and refs/remotes pull in nearly 40,000 ref names in source, making completion in common contexts like git checkout <tab> unusable.

This diff updates git bash completion of refs to allow customization of behavior via git config variables, as described in doc comment at top of git-completion.bash file. With these config variables, completion of ref names can be limited to only refs/heads, dramatically increasing the speed of git checkout <tab>.

[jakesylvestre]

I think this repo is dead. I'd close this PR

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