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>

git-gui.sh

@@ -1171,7 +1171,7 @@ File [short_path $path] cannot be committed by this program.
 		}
 		}
 	}
-	if {!$files_ready} {
+	if {!$files_ready && ![string match *merge $curType]} {
 		info_popup {No changes to commit.
 
 You must add at least 1 file before you can commit.