get_sha1_basic(): corner case ambiguity fix

get_sha1_basic(): corner case ambiguity fix

When .git/refs/heads/frotz and .git/refs/tags/frotz existed, and
the object name stored in .git/refs/heads/frotz were corrupt, we
ended up picking tags/frotz without complaining.  Worse yet, if
the corrupt .git/refs/heads/frotz was more than 40 bytes and
began with hexadecimal characters, it silently overwritten the
initial part of the returned result.

This commit adds a couple of tests to demonstrate these cases,
with a fix.

Signed-off-by: Junio C Hamano <junkio@cox.net>