Do not use memcmp(sha1_1, sha1_2, 20) with hardcoded length.

Do not use memcmp(sha1_1, sha1_2, 20) with hardcoded length.

Introduces global inline:

	hashcmp(const unsigned char *sha1, const unsigned char *sha2)

Uses memcmp for comparison and returns the result based on the length of
the hash name (a future runtime decision).

Acked-by: Alex Riesen <redacted>
Signed-off-by: David Rientjes <redacted>
Signed-off-by: Junio C Hamano <redacted>