Merge branch 'js/rebase-i-allow-drop-on-a-merge'

During interactive rebase, using 'drop' on a merge commit lead to
an error, which was incorrect.

* js/rebase-i-allow-drop-on-a-merge:
  rebase -i: permit 'drop' of a merge commit

sequencer.c

@@ -2721,6 +2721,7 @@ static int check_merge_commit_insn(enum todo_command command)
 		return error(_("cannot squash merge commit into another commit"));
 
 	case TODO_MERGE:
+	case TODO_DROP:
 		return 0;
 
 	default:

t/t3404-rebase-interactive.sh

@@ -2263,6 +2263,7 @@ test_expect_success 'non-merge commands reject merge commits' '
 	edit $oid
 	fixup $oid
 	squash $oid
+	drop $oid # acceptable, no advice
 	EOF
 	(
 		set_replace_editor todo &&