Integrate hash algorithm support with repo setup

Integrate hash algorithm support with repo setup

In future versions of Git, we plan to support an additional hash
algorithm.  Integrate the enumeration of hash algorithms with repository
setup, and store a pointer to the enumerated data in struct repository.
Of course, we currently only support SHA-1, so hard-code this value in
read_repository_format.  In the future, we'll enumerate this value from
the configuration.

Add a constant, the_hash_algo, which points to the hash_algo structure
pointer in the repository global.  Note that this is the hash which is
used to serialize data to disk, not the hash which is used to display
items to the user.  The transition plan anticipates that these may be
different.  We can add an additional element in the future (say,
ui_hash_algo) to provide for this case.

Include repository.h in cache.h since we now need to have access to
these struct and variable definitions.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>