commit-msg: Remove MERGE_HEAD check from msg_is_merge

When using "git commit --amend" to edit a merge commit message the
MERGE_HEAD is long gone.  Also, some Git versions remove this file
before the commit-msg hook is called.  In either case we should still
allow long summary lines that start in 'Merge ' just as we do for
'Revert '.

commit-msg

@@ -44,7 +44,6 @@ die() {
 # Check the commit message layout with a simple state machine.
 
 msg_is_merge() {
-	test -f "$GIT_DIR/MERGE_HEAD" &&
 	echo "$line" | grep "^Merge " >/dev/null 2>&1
 }