Add a check for fast-forward-ability

With fast-forward merges (!274 (merged)), a check should check for this support. The merge action is the ultimate check on it since the check is inherently temporal in nature.

Things to consider:

• Is a fast-forward merge supported?
• Is the target branch ahead of the requested branch?

I don't think this is implementable in git-checks, so it has to be a check provided by this crate.

Cc: @brad.king