gitweb: Fix handling of $file_name in feed generation

gitweb: Fix handling of $file_name in feed generation

The commit b6093a5c, by Robert Fitzsimons:
  "gitweb: Change atom, rss actions to use parse_commits."
forgot to pass $file_name parameter to parse_commits subroutine.

If git_feed is provided a file name, it ought to show only the history
affecting that file or a directory.  The title was being set
correctly, but all commits from history were being shown.

Signed-off-by: Steven Walter <stevenrwalter@gmail.com>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>