Add EmptyMessage commit-msg hook

[jawshooah]

This hook will fail if the commit message is empty or contains only whitespace. Enabled by default, as I suspect this is something most users would never do intentionally.

Refs #201

[lencioni]

👍

[sds]

Not entirely sure this is necessary, as git seems to already prevent you from creating empty commits.

$ mkdir test
$ cd test
$ git init
$ git commit --allow-empty -m " "
Aborting commit due to empty commit message.

[lencioni]

Oh yeah, that's a good point. I wonder if there are any versions in popular use where this isn't the case?

[jawshooah]

$ git commit --allow-empty --allow-empty-message -m " "

[lencioni]

Oh, looks like you can pass --allow-empty-message in newer versions, so this still probably makes sense.

[sds]

Why there exists an --allow-empty-message flag in the first place is a bit of a head scratcher.

In any case, merged in 435d982.