Verify Git commands (implements #202)

@charlierudolph this is an early preview. I'd like your thoughts before I add this everywhere.
Also please explain the regex. Shouldn't it be the following

```ruby
/^\[(.+)\] (.+)$/
```

Looks like git-commit also has the following output
```
[<branch> <sha>] <message>
```
The two difference we know we will have are
* there will no space in the brackets
* the first word outside of the brackets will be git. 

So maybe the following is a better regex:

```ruby
/^\[([^ ]+)\] (git .+)$/
```