sha1-file: provide functions to look up hash algorithms

sha1-file: provide functions to look up hash algorithms

There are several ways we might refer to a hash algorithm: by name, such
as in the config file; by format ID, such as in a pack; or internally,
by a pointer to the hash_algos array.  Provide functions to look up hash
algorithms based on these various forms and return the internal constant
used for them.  If conversion to another form is necessary, this
internal constant can be used to look up the proper data in the
hash_algos array.

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