clone: refuse to clone if --branch points to bogus ref

It's possible that users make a typo in the branch name. Stop and let
users recheck. Falling back to remote's HEAD is not documented any
way.

Except when using remote helper, the pack has not been transferred at
this stage yet so we don't waste much bandwidth.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/clone.c

@@ -518,8 +518,7 @@ static void update_head(const struct ref *our, const struct ref *remote,
 	} else if (remote) {
 		/*
 		 * We know remote HEAD points to a non-branch, or
-		 * HEAD points to a branch but we don't know which one, or
-		 * we asked for a specific branch but it did not exist.
+		 * HEAD points to a branch but we don't know which one.
 		 * Detach HEAD in all these cases.
 		 */
 		update_ref(msg, "HEAD", remote->old_sha1,
@@ -816,12 +815,9 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 			our_head_points_at =
 				find_remote_branch(mapped_refs, option_branch);
 
-			if (!our_head_points_at) {
-				warning(_("Remote branch %s not found in "
-					"upstream %s, using HEAD instead"),
-					option_branch, option_origin);
-				our_head_points_at = remote_head_points_at;
-			}
+			if (!our_head_points_at)
+				die(_("Remote branch %s not found in upstream %s"),
+				    option_branch, option_origin);
 		}
 		else
 			our_head_points_at = remote_head_points_at;

t/t5500-fetch-pack.sh

@@ -282,13 +282,6 @@ test_expect_success 'clone shallow object count' '
 	test_cmp count3.expected count3.actual
 '
 
-test_expect_success 'clone shallow with nonexistent --branch' '
-	git clone --depth 1 --branch Z "file://$(pwd)/." shallow4 &&
-	GIT_DIR=shallow4/.git git rev-parse HEAD >actual &&
-	git rev-parse HEAD >expected &&
-	test_cmp expected actual
-'
-
 test_expect_success 'clone shallow with detached HEAD' '
 	git checkout HEAD^ &&
 	git clone --depth 1 "file://$(pwd)/." shallow5 &&

t/t5706-clone-branch.sh

@@ -57,12 +57,8 @@ test_expect_success 'clone -b does not munge remotes/origin/HEAD' '
 	)
 '
 
-test_expect_success 'clone -b with bogus branch chooses HEAD' '
-	git clone -b bogus parent clone-bogus &&
-	(cd clone-bogus &&
-	 check_HEAD master &&
-	 check_file one
-	)
+test_expect_success 'clone -b with bogus branch' '
+	test_must_fail git clone -b bogus parent clone-bogus
 '
 
 test_done