Verify Git commands (implements #202)

@charlierudolph this is an early preview. I'd like your thoughts before I add this everywhere.
I am currently parsing the textual output of the Git Town command. It has this format:

**[master]  git checkout feature**
Switched to branch 'feature'
Your branch is up-to-date with 'origin/feature'.

**[feature] git pull**
Already up-to-date.

I have rewritten the regex in expanded mode with explanations. Let's discuss it in the code.