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>

gitweb/gitweb.perl

@@ -1058,7 +1058,7 @@ sub format_extended_diff_header_line {
 
 # format from-file/to-file diff header
 sub format_diff_from_to_header {
-	my ($from_line, $to_line, $diffinfo, $from, $to) = @_;
+	my ($from_line, $to_line, $diffinfo, $from, $to, @parents) = @_;
 	my $line;
 	my $result = '';
 
@@ -1084,7 +1084,17 @@ sub format_diff_from_to_header {
 		for (my $i = 0; $i < $diffinfo->{'nparents'}; $i++) {
 			if ($from->{'href'}[$i]) {
 				$line = '--- ' .
-				        ($i+1) . "/" .
+				        $cgi->a({-href=>href(action=>"blobdiff",
+				                             hash_parent=>$diffinfo->{'from_id'}[$i],
+				                             hash_parent_base=>$parents[$i],
+				                             file_parent=>$from->{'file'}[$i],
+				                             hash=>$diffinfo->{'to_id'},
+				                             hash_base=>$hash,
+				                             file_name=>$to->{'file'}),
+				                 -class=>"path",
+				                 -title=>"diff" . ($i+1)},
+				                $i+1) .
+				        '/' .
 				        $cgi->a({-href=>$from->{'href'}[$i], -class=>"path"},
 				                esc_path($from->{'file'}[$i]));
 			} else {
@@ -3042,7 +3052,8 @@ sub git_patchset_body {
 		#assert($patch_line =~ m/^\+\+\+/) if DEBUG;
 
 		print format_diff_from_to_header($last_patch_line, $patch_line,
-		                                 $diffinfo, \%from, \%to);
+		                                 $diffinfo, \%from, \%to,
+		                                 @hash_parents);
 
 		# the patch itself
 	LINE: