merge-resolve: avoid dashed call

We started deprecating the dashed form a long time ago, advertising the
fact that dashed invocations of Git commands are deprecated for several
major versions.

Yet we still used them ourselves.

With ashes on our heads, we now start to finally get rid of those calls.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

git-merge-resolve.sh

@@ -45,7 +45,7 @@ then
 	exit 0
 else
 	echo "Simple merge failed, trying Automatic merge."
-	if git-merge-index -o git-merge-one-file -a
+	if git merge-index -o git-merge-one-file -a
 	then
 		exit 0
 	else