branch tests: test for errno propagating on failing read

Add a test for "git branch" to cover the case where .git/refs is
symlinked. To check availability, refs_verify_refname_available() will
run refs_read_raw_ref() on each prefix, leading to a read() from
.git/refs (which is a directory).

It would probably be more robust to re-issue the lstat() as a normal
stat(), in which case, we would fall back to the directory case, but
for now let's just test for the existing behavior as-is. This test
covers a regression in a commit that only ever made it to "next", see
[1].

1. http://lore.kernel.org/git/pull.1068.git.git.1629203489546.gitgitgadget@gmail.com

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3200-branch.sh

@@ -731,6 +731,28 @@ test_expect_success SYMLINKS 'git branch -m u v should fail when the reflog for
 	test_must_fail git branch -m u v
 '
 
+test_expect_success SYMLINKS 'git branch -m with symlinked .git/refs' '
+	test_when_finished "rm -rf subdir" &&
+	git init --bare subdir &&
+
+	rm -rfv subdir/refs subdir/objects subdir/packed-refs &&
+	ln -s ../.git/refs subdir/refs &&
+	ln -s ../.git/objects subdir/objects &&
+	ln -s ../.git/packed-refs subdir/packed-refs &&
+
+	git -C subdir rev-parse --absolute-git-dir >subdir.dir &&
+	git rev-parse --absolute-git-dir >our.dir &&
+	! test_cmp subdir.dir our.dir &&
+
+	git -C subdir log &&
+	git -C subdir branch rename-src &&
+	git rev-parse rename-src >expect &&
+	git -C subdir branch -m rename-src rename-dest &&
+	git rev-parse rename-dest >actual &&
+	test_cmp expect actual &&
+	git branch -D rename-dest
+'
+
 test_expect_success 'test tracking setup via --track' '
 	git config remote.local.url . &&
 	git config remote.local.fetch refs/heads/*:refs/remotes/local/* &&