Add CLI flag to clean format-patch titles and messages

[bluekeyes]

Applying a patch created by git format-patch works, but leaves a kind of messy result:

• The title of the commit and the PR title contain a [PATCH] prefix
• The body of the commit and the PR body contain a summary of the changed files (content that appears after the --- in the patch message)

It would be nice to have a flag that attempted to strip this stuff out of the title and body. If I remember correctly, the -k flag for format-patch and am does something similar.

[bluekeyes]

I think this was fixed by bluekeyes/go-gitdiff#19 and bluekeyes/go-gitdiff#17. The extra information is no longer included in the fields used to generate commit and PR messages.