builtin/receive-pack: avoid generic function name hmac()

builtin/receive-pack: avoid generic function name hmac()

fabec2c5c3 (builtin/receive-pack: switch to use the_hash_algo, 2019-08-18)
renames hmac_sha1 to hmac, as it was updated to use the hash function used
by git (which won't be sha1 in the future).

hmac() is provided by NetBSD >= 8 libc and therefore conflicts as shown by :

builtin/receive-pack.c:421:13: error: conflicting types for 'hmac'
 static void hmac(unsigned char *out,
             ^~~~
In file included from ./git-compat-util.h:172:0,
                 from ./builtin.h:4,
                 from builtin/receive-pack.c:1:
/usr/include/stdlib.h:305:10: note: previous declaration of 'hmac' was here
 ssize_t  hmac(const char *, const void *, size_t, const void *, size_t, void *,
          ^~~~

Rename it again to hmac_hash to reflect it will use the git's defined hash
function and avoid the conflict, while at it update a comment to better
describe the HMAC function that was used.

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>