gitweb: Fix split patches output (e.g. file to symlink)

gitweb: Fix split patches output (e.g. file to symlink)

Do not replace /dev/null in two-line from-file/to-file diff header for
split patches ("split" patch mean more than one patch per one
diff-tree raw line) by a/file or b/file link.

Split patches differ from pair of deletion/creation patch in git diff
header: both a/file and b/file are hyperlinks, in all patches in a
split.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>