gitweb: Support comparing blobs (files) with different names

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>