Allow the Unix epoch to be a valid commit date

It is common practice to use the Unix epoch as a fallback date
when a suitable date is not available.  This is true of git svn
and possibly other importing tools that import non-git history
into git.

Instead of clobbering established strtoul() error reporting
semantics with our own, preserve the strtoul() error value
of ULONG_MAX for fsck.c to handle.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -50,7 +50,6 @@ struct commit *lookup_commit(const unsigned char *sha1)
 
 static unsigned long parse_commit_date(const char *buf, const char *tail)
 {
-	unsigned long date;
 	const char *dateptr;
 
 	if (buf + 6 >= tail)
@@ -73,10 +72,7 @@ static unsigned long parse_commit_date(const char *buf, const char *tail)
 	if (buf >= tail)
 		return 0;
 	/* dateptr < buf && buf[-1] == '\n', so strtoul will stop at buf-1 */
-	date = strtoul(dateptr, NULL, 10);
-	if (date == ULONG_MAX)
-		date = 0;
-	return date;
+	return strtoul(dateptr, NULL, 10);
 }
 
 static struct commit_graft **commit_graft;

fsck.c

@@ -229,7 +229,7 @@ static int fsck_commit(struct commit *commit, fsck_error error_func)
 	struct commit_graft *graft;
 	int parents = 0;
 
-	if (!commit->date)
+	if (commit->date == ULONG_MAX)
 		return error_func(&commit->object, FSCK_ERROR, "invalid author/committer line");
 
 	if (memcmp(buffer, "tree ", 5))