bundle: Accept prerequisites without commit messages

While explicitly stating that the commit message in a prerequisite
line is optional, we required all lines with 40 or more characters
to contain a space after the object name, bailing out if a line
consisted of an object name only. This was to allow bundling a
history to a commit without an message, but the code forgot that it
already called rtrim() to remove that whitespace.

As a workaround, only check for SP when the line has more than 40
characters.

Signed-off-by: Lukas Fleischer <git@cryptocrack.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bundle.c

@@ -57,7 +57,7 @@ static int parse_bundle_header(int fd, struct bundle_header *header,
 		 * followed by SP and subject line.
 		 */
 		if (get_sha1_hex(buf.buf, sha1) ||
-		    (40 <= buf.len && !isspace(buf.buf[40])) ||
+		    (buf.len > 40 && !isspace(buf.buf[40])) ||
 		    (!is_prereq && buf.len <= 40)) {
 			if (report_path)
 				error(_("unrecognized header: %s%s (%d)"),