commit-msg: Allow long first lines starting in "Revert "

Revert commits always have longer first lines than the commit they
revert.  Do not reject those that happen to go over the threshold.  It
is much simpler if the "git revert" command creates the commit without
error.

commit-msg

@@ -41,6 +41,10 @@ msg_is_merge() {
 	echo "$line" | grep "^Merge " >/dev/null 2>&1
 }
 
+msg_is_revert() {
+	echo "$line" | grep "^Revert " >/dev/null 2>&1
+}
+
 msg_first() {
 	len=$(echo -n "$line" | wc -c)
 	if test $len -eq 0; then
@@ -51,7 +55,7 @@ msg_first() {
 --------
 '"$line"'
 --------'
-	elif test $len -gt 78 && ! msg_is_merge; then
+	elif test $len -gt 78 && ! msg_is_merge && ! msg_is_revert; then
 		die 'The first line may be at most 78 characters:
 ------------------------------------------------------------------------------
 '"$line"'