Different tab completion rules for different Git versions

[georgebrock]

The current tab completion file is based on the manual pages from Git 2.13.1. Newer versions of Git could introduce options and arguments that we won't be able to complete. Conversely, we will offer completions to users of older versions of Git that they can't actually use.

[georgebrock]

Bumping this from v0.14 to v1.0: this is still an important feature, but I don't want to delay releasing some of the big tab completion improvements that we've done recently for something that's more complex, and arguably less important.