refspec: let callers pass in hash algorithm when parsing items

refspec: let callers pass in hash algorithm when parsing items

When parsing a refspec item we need to know about the hash algorithm
used by the repository so that we can decide whether or not a given
string is an exact object ID. We use `the_hash_algo` for this, which
makes the code implicitly depend on `the_repository`.

Refactor `refspec_item_init_fetch()`, `refspec_item_init_push()` and
`valid_fetch_refspec()` so that callers have to pass in the hash
algorithm explicitly and adapt callers accordingly. For now, all of
the callers simply pass `the_hash_algo`, so there is no change in
behaviour.

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