gitweb: Add support for hash_parent_base parameter for blobdiffs

gitweb: Add support for hash_parent_base parameter for blobdiffs

Add support for hash_parent_base in input validation part and in
href() function.  Add proper hash_parent_base to all calls to blobdiff
and blobdiff_plain action URLs. Use hash_parent_base as hash_base for
blobs of hash_parent.

To be used in future rewrite of git_blobdiff and git_blobdiff_plain.

While at it, move project before action in ordering CGI parameters in
href().

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>