branch tests: test for errno propagating on failing read

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>