refs: make rev-parse --quiet actually quiet

When a reflog is deleted, e.g. when "git stash" clears its stashes,
"git rev-parse --verify --quiet" dies:

	fatal: Log for refs/stash is empty.

The reason is that the get_sha1() code path does not allow us
to suppress this message.

Pass the flags bitfield through get_sha1_with_context() so that
read_ref_at() can suppress the message.

Use get_sha1_with_context1() instead of get_sha1() in rev-parse
so that the --quiet flag is honored.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
stash: prefer --quiet over shell redirection of the standard error stream

Use `git rev-parse --verify --quiet` instead of redirecting
stderr to /dev/null.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>