clone --branch: refuse to clone if upstream repo is empty

Since 920b691f (clone: refuse to clone if --branch
points to bogus ref) we refuse to clone with option
"-b" if the specified branch does not exist in the
(non-empty) upstream. If the upstream repository is empty,
the branch doesn't exist, either. So refuse the clone too.
Reported-by: NRobert Mitwicki <robert.mitwicki@opensoftware.pl>
Signed-off-by: NRalf Thielow <ralf.thielow@gmail.com>
Acked-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>

builtin/clone.c

@@ -946,6 +946,10 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 			our_head_points_at = remote_head_points_at;
 	}
 	else {
+		if (option_branch)
+			die(_("Remote branch %s not found in upstream %s"),
+					option_branch, option_origin);
+
 		warning(_("You appear to have cloned an empty repository."));
 		mapped_refs = NULL;
 		our_head_points_at = NULL;

t/t5706-clone-branch.sh

@@ -20,7 +20,9 @@ test_expect_success 'setup' '
 	 echo one >file && git add file && git commit -m one &&
 	 git checkout -b two &&
 	 echo two >file && git add file && git commit -m two &&
-	 git checkout master)
+	 git checkout master) &&
+	mkdir empty &&
+	(cd empty && git init)
 '
 
 test_expect_success 'vanilla clone chooses HEAD' '
@@ -61,4 +63,8 @@ test_expect_success 'clone -b with bogus branch' '
 	test_must_fail git clone -b bogus parent clone-bogus
 '
 
+test_expect_success 'clone -b not allowed with empty repos' '
+	test_must_fail git clone -b branch empty clone-branch-empty
+'
+
 test_done