cache: make hashcmp and hasheq work with larger hashes

cache: make hashcmp and hasheq work with larger hashes

In 183a638b7d ("hashcmp: assert constant hash size", 2018-08-23), we
modified hashcmp to assert that the hash size was always 20 to help it
optimize and inline calls to memcmp.  In a future series, we replaced
many calls to hashcmp and oidcmp with calls to hasheq and oideq to
improve inlining further.

However, we want to support hash algorithms other than SHA-1, namely
SHA-256.  When doing so, we must handle the case where these values are
32 bytes long as well as 20.  Adjust hashcmp to handle two cases:
20-byte matches, and maximum-size matches.  Therefore, when we include
SHA-256, we'll automatically handle it properly, while at the same time
teaching the compiler that there are only two possible options to
consider.  This will allow the compiler to write the most efficient
possible code.

Copy similar code into hasheq and perform an identical transformation.
At least with GCC 8.2.0, making hasheq defer to hashcmp when there are
two branches prevents the compiler from inlining the comparison, while
the code in this patch is inlined properly.  Add a comment to avoid an
accidental performance regression from well-intentioned refactoring.

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