t0610: execute git-pack-refs(1) with specified umask

t0610: execute git-pack-refs(1) with specified umask

The tests for git-pack-refs(1) with the `core.sharedRepository` config
execute git-pack-refs(1) outside of the shell that has the expected
umask set. This is wrong because we want to test the behaviour of that
command with different umasks. The issue went unnoticed because most
distributions have a default umask of 0022, and we only ever test with
`--shared=true`, which re-adds the group write bit.

Fix the issue by moving git-pack-refs(1) into the umask'd shell and add
a bunch of test cases that exercise behaviour more thoroughly.

Note that we drop the check for whether `core.sharedRepository` was set
to the correct value to make the test setup a bit easier. We should be
able to rely on git-init(1) doing its thing correctly. Furthermore, to
help readability, we convert tests that pass `--shared=true` to instead
pass the equivalent `--shared=group`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>