gitweb: Use feed link according to current view

gitweb: Use feed link according to current view

Michael G. Noll said in comments to the "Switching my code repository from
Subversion (SVN) to git" article (http://tinyurl.com/37v67l) in his "My
digital moleskine" blog, that one of the things he is missing in gitweb
from SVN::Web is an RSS feed with news/information of the current view
(including RSS feed for single file or directory).

This is not exactly true, as since refactoring feed generation in af6feeb
(gitweb: Refactor feed generation, make output prettier, add Atom feed,
2006-11-19), gitweb can generate feeds (RSS or Atom) for history of a
given branch, history limited to a given directory, or history of a given
file.  Nevertheless this required handcrafting the URL to get wanted RSS
feed.

This commit makes gitweb select feed links in the HTML header and in
page footer depending on current view (action).  It is more elaborate,
and I guess more correct, than simple patch adding $hash ('h')
parameter to *all* URLs, including feed links, by Jean-Baptiste Quenot

  Subject: [PATCH] gitweb: Add hash parameter in feed URL when a hash
           is specified in the current request
  Message-ID: <ae63f8b50803211138y6355fd11pa64cda50a1f53011@mail.gmail.com>

If $hash ('h') or $hash_base ('hb') parameter is a branch name
(i.e. it starts with 'refs/heads/'; all generated URLs use this form
to discriminate between tags and heads), it is used in feed URLs; if
$file_name ('f') is defined, it is used in feed URLs.  Feed title is
set according to the kind of web feed: it is either 'log' for generic
feed, 'log of <branch>', 'history of <filename>' for generic history
(using implicit or explicit HEAD, i.e. current branch) or 'history of
<filename> on <branch>'.

There are special cases: 'heads' and 'forks' views should use OPML
providing list of available feeds; 'tags' probably also should use
OPML; there is no web feed equivalent to 'search' view.  Currently all
those cases fallback to (show) default feed.  Such feed link uses
"generic" class, and is shown in slightly lighter color for
distinction.

Currently feed can have but one starting point, and does not support
negative (exclude) commit arguments.  Therefore for now for *diff
views it is chosen that feed follow the "to" part: to-name, to-commit
for 'blobdiff', 'treediff' and 'commitdiff' views.

Generating parameters for href() for feed link was separated
(refactored) into get_feed_info() subroutine.

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