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 <raa.lkml@gmail.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

cache.h

@@ -214,6 +214,10 @@ static inline int is_null_sha1(const unsigned char *sha1)
 {
 	return !memcmp(sha1, null_sha1, 20);
 }
+static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
+{
+	return memcmp(sha1, sha2, 20);
+}
 
 int git_mkstemp(char *path, size_t n, const char *template);