refspec: stop depending on `the_repository`

refspec: stop depending on `the_repository`

The only remaining user of `the_hash_algo` in "refspec.c" is
`refspec_append()`, which needs to know the hash algorithm so that it
can parse the appended refspec item. In contrast to the functions
adapted in the preceding commit, this function always operates on a
`struct refspec`. As that structure is expected to only ever contain
refspecs that all use the same hash function it doesn't make sense
though to adapt each caller.

Instead, adapt the structure itself so that it gets initialized with a
hash function and use that hash function to parse new refspec items.
Adapt callers accordingly.

This removes the final dependency on the global repository variable in
"refspec.c", so we can drop `USE_THE_REPOSITORY_VARIABLE`.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>