gitweb: fix 'Use of uninitialized value' error in href()

Equality between file_parent and file_name was being checked without a
preliminary check for existence of the parameters.

Fix by wrapping the equality check in appropriate if (defined ...),
rearranging the lines to prevent excessive length.

Signed-off-by: Giuseppe Bilotta <redacted>
Acked-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>