validate_headref: use get_oid_hex for detached HEADs

If a candidate HEAD isn't a symref, we check that it
contains a viable sha1. But in a post-sha1 world, we should
be checking whether it has any plausible object-id.

We can do that by switching to get_oid_hex().

Note that both before and after this patch, we only check
for a plausible object id at the start of the file, and then
call that good enough.  We ignore any content _after_ the
hex, so a string like:

  0123456789012345678901234567890123456789 foo

is accepted. Though we do put extra bytes like this into
some pseudorefs (e.g., FETCH_HEAD), we don't typically do so
with HEAD. We could tighten this up by using parse_oid_hex(),
like:

  if (!parse_oid_hex(buffer, &oid, &end) &&
      *end++ == '\n' && *end == '\0')
          return 0;

But we're probably better to remain on the loose side. We're
just checking here for a plausible-looking repository
directory, so heuristics are acceptable (if we really want
to be meticulous, we should use the actual ref code to parse
HEAD).
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

path.c

@@ -638,7 +638,7 @@ int validate_headref(const char *path)
 	struct stat st;
 	char buffer[256];
 	const char *refname;
-	unsigned char sha1[20];
+	struct object_id oid;
 	int fd;
 	ssize_t len;
 
@@ -679,7 +679,7 @@ int validate_headref(const char *path)
 	/*
 	 * Is this a detached HEAD?
 	 */
-	if (!get_sha1_hex(buffer, sha1))
+	if (!get_oid_hex(buffer, &oid))
 		return 0;
 
 	return -1;