apply: fix uninitialized hash function

apply: fix uninitialized hash function

"git apply" can work outside a repository as a better "GNU patch",
but when it does so, it still assumed that it can access
the_hash_algo, which is no longer true in the new world order.

Make sure we explicitly fall back to SHA-1 algorithm for backward
compatibility.

It is of dubious value to make this configurable to other hash
algorithms, as the code does not use the_hash_algo for hashing
purposes when working outside a repository (which is how
the_hash_algo is left to NULL)---it is only used to learn the max
length of the hash when parsing the object names on the "index"
line, but failing to parse the "index" line is not a hard failure,
and the program does not support operations like applying binary
patches and --3way fallback that requires object access outside a
repository.

Signed-off-by: Junio C Hamano <gitster@pobox.com>