gitweb: Support comparing blobs (files) with different names

Fix the bug that caused "blobdiff" view called with new style URI
for a rename with change diff to be show as new (added) file diff.

New style URI for "blobdiff" for rename means with $hash_base ('hb') and
$hash_parent_base ('hpb') paramaters denoting tree-ish (usually commit)
of a blobs being compared, together with both $file_name ('f') and
$file_parent ('fp') parameters.

It is done by adding $file_parent ('fp') to the path limiter, meaning
that diff command becomes:

	git diff-tree [options] hpb hb -- fp f

Other option would be finding hash of a blob using git_get_hash_by_path
subroutine and comparing blobs using git-diff, or using extended SHA-1
syntax and compare blobs using git-diff:

	git diff [options] hpb:fp hp:f

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

gitweb/gitweb.perl

@@ -3885,7 +3885,7 @@ sub git_blobdiff {
 			# read raw output
 			open $fd, "-|", git_cmd(), "diff-tree", '-r', @diff_opts,
 				$hash_parent_base, $hash_base,
-				"--", $file_name
+				"--", (defined $file_parent ? $file_parent : ()), $file_name
 				or die_error(undef, "Open git-diff-tree failed");
 			@difftree = map { chomp; $_ } <$fd>;
 			close $fd
@@ -3935,7 +3935,7 @@ sub git_blobdiff {
 		# open patch output
 		open $fd, "-|", git_cmd(), "diff-tree", '-r', @diff_opts,
 			'-p', $hash_parent_base, $hash_base,
-			"--", $file_name
+			"--", (defined $file_parent ? $file_parent : ()), $file_name
 			or die_error(undef, "Open git-diff-tree failed");
 	}