packfile: abstract away hash constant values

packfile: abstract away hash constant values

There are several instances of the constant 20 and 20-based values in
the packfile code.  Abstract away dependence on SHA-1 by using the
values from the_hash_algo instead.

Use unsigned values for temporary constants to provide the compiler with
more information about what kinds of values it should expect.

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