Improve completer speed for git checkout

[hongquan]

Improvement:

• Faster to give result (calling git less, telling git to return simpler-to-parse output).
• Context aware. For example:
  • After git checkout branch-name, the rest arguments must be file paths (git checkout only accepts one "branch").
  • After -- are file paths. git checkout a-branch -- a-file

The current completer for git checkout is slow because:

• Running git too many times.
• Parse many results and end up not use.

[fdncred]

LGTM! Thanks