Merge branch 'cb/maint-no-double-merge'

* cb/maint-no-double-merge:
  refuse to merge during a merge

builtin-merge.c

@@ -836,8 +836,11 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	struct commit_list **remotes = &remoteheads;
 
 	setup_work_tree();
+	if (file_exists(git_path("MERGE_HEAD")))
+		die("You have not concluded your merge. (MERGE_HEAD exists)");
 	if (read_cache_unmerged())
-		die("You are in the middle of a conflicted merge.");
+		die("You are in the middle of a conflicted merge."
+				" (index unmerged)");
 
 	/*
 	 * Check if we are _not_ on a detached HEAD, i.e. if there is a

t/t3030-merge-recursive.sh

@@ -276,6 +276,9 @@ test_expect_success 'fail if the index has unresolved entries' '
 
 	test_must_fail git merge "$c5" &&
 	test_must_fail git merge "$c5" 2> out &&
+	grep "You have not concluded your merge" out &&
+	rm -f .git/MERGE_HEAD &&
+	test_must_fail git merge "$c5" 2> out &&
 	grep "You are in the middle of a conflicted merge" out
 
 '