gitweb: Make feed entries point to commitdiff view

gitweb: Make feed entries point to commitdiff view

Change feeds entries (feeds items) from pointing (linking) to 'commit'
view to pointing to 'commitdiff' view.

First, feed entries have whatchanged-like list of files which were
modified in a commit, so 'commitdiff' view more naturally reflects
feed entry (is more naturally alternate / extended version of a feed
item). Second, this way the patches are shown directly and code review
is done more easily via watching feeds.

[jn: Rewritten commit message]

Signed-off-by: Florian La Roche <laroche@redhat.com>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>