builtin/rev-parse: allow shortening to more than 40 hex characters

builtin/rev-parse: allow shortening to more than 40 hex characters

The `--short=` option for git-rev-parse(1) allows the user to specify
to how many characters object IDs should be shortened to. The option is
broken though for SHA256 repositories because we set the maximum allowed
hash size to `the_hash_algo->hexsz` before we have even set up the repo.
Consequently, `the_hash_algo` will always be SHA1 and thus we truncate
every hash after at most 40 characters.

Fix this by accessing `the_hash_algo` only after we have set up the
repo.

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