git-gui: Allow committing empty merges

Johannes Sixt noticed that git-gui would not let the user commit
a merge created by `git merge -s ours` as the ours strategy does
not alter the tree (that is HEAD^1^{tree} = HEAD^{tree} after the
merge).  The same issue arises from amending such a merge commit.

We now permit an empty commit (no changed files) if we are doing
a merge commit.  Core Git does this with its command line based
git-commit tool, so it makes sense for the GUI to do the same.

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