Divorce leading @ symbols from identifiers in commit messages

There's no reason to believe that, e.g. @sgolemon on github is also
@sgolemon on twitter.  Add a space to avoid spamming innocent bystanders.

I considered simply stripping the @, but that might change the meaning
of other uses, such as simple shorthand.

Therefore:
  "Ooops, I broke this @12:07, thanks @sgolemon!"

becomes:
  "Ooops, I broke this @ 12:07, thanks @ sgolemon!"