git-gui: Teach sign off to be more intelligent.

git-gui: Teach sign off to be more intelligent.

When we sign off on a commit we want to add a blank line between
whatever is in the commit buffer and the new Signed-off-by line,
unless there already is a Signed-off-by (or Acked-by) tag at the end
of the buffer already.  This change makes us do the right thing more
often.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>