Use help buffers in commit and merge scripts.

Use help buffers in commit and merge scripts.

The previous attempt to override Vim 7.4's window closing paranoia
didn't work that well.  Closing the scratch buffer would result in
the commit message window being lost without saving changes.

The latest scheme is to mark the scratch buffer as a help buffer,
despite the documentation saying we aren't supposed to do that.
The result is that it will automatically be closed when it's the
last buffer standing, which is very close to what we want.