hashmap: convert sha1hash() to oidhash()

There are no callers left of sha1hash() that do not simply pass the
"hash" member of a "struct object_id". Let's get rid of the outdated
sha1-specific function and provide one that operates on the whole struct
(even though the technique, taking the first few bytes of the hash, will
remain the same).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

patch-ids.c

@@ -83,7 +83,7 @@ static int init_patch_id_entry(struct patch_id *patch,
 	if (commit_patch_id(commit, &ids->diffopts, &header_only_patch_id, 1, 0))
 		return -1;
 
-	hashmap_entry_init(patch, sha1hash(header_only_patch_id.hash));
+	hashmap_entry_init(patch, oidhash(&header_only_patch_id));
 	return 0;
 }