gitweb: Add links to blobdiffs in from-file/to-file header for merges

gitweb: Add links to blobdiffs in from-file/to-file header for merges

Add links to diff to file ('blobdiff' view) for each of individual
versions of the file in a merge commit to the from-file/to-file header
in the patch part of combined 'commitdiff' view for merges.

The from-file/to-file header for combined diff now looks like:

  --- _1_/_git-gui/git-gui.sh_
  --- _2_/_git-gui.sh_
  +++ b/_git-gui/git-gui.sh_

where _<filename>_ link is link to appropriate version of a file
('blob' view), and _<n>_ is link to respective diff to mentioned
version of a file ('blobdiff' view). There is even hint provided in
the form of title attribute.

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