gitweb: Finish restoring "blob" links in git_difftree_body

gitweb: Finish restoring "blob" links in git_difftree_body

This finishes work started by commit 4777b0141a4812177390da4b6ebc9d40ac3da4b5
  "gitweb: Restore object-named links in item lists"
by Petr Baudis. It brings back rest of "blob" links in difftree-raw
like part of "commit" and "commitdiff" views, namely in
git_difftree_body subroutine.

Now the td.link table cell has the following links:
 * link to diff ("blobdiff" view) in "commit" view, if applicable
   (there is no link to uninteresting creation/deletion diff), or
   link to patch anchor in "commitdiff" view.
 * link to current version of file ("blob" view), with the obvious
   exception of file deletion, where it is link to the parent
   version.
 * link to "blame" view, if it is enabled, and file was not just
   created (i.e. it has any history).
 * link to history of the file ("history" view), again with sole
   exception of the case of new file.

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