parse-options-cb: only abbreviate hashes when hash algo is known

parse-options-cb: only abbreviate hashes when hash algo is known

The `OPT__ABBREV()` option can be used to add an option that abbreviates
object IDs. When given a length longer than `the_hash_algo->hexsz`, then
it will instead set the length to that maximum length.

It may not always be guaranteed that we have `the_hash_algo` initialized
properly as the hash algorithm can only be set up after we have set up
`the_repository`. In that case, the hash would always be truncated to
the hex length of SHA1, which may not be what the user desires.

In practice it's not a problem as all commands that use `OPT__ABBREV()`
also have `RUN_SETUP` set and thus cannot work without a repository.
Consequently, both `the_repository` and `the_hash_algo` would be
properly set up.

Regardless of that, harden the code to not truncate the length when we
didn't set up a repository.

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