gitweb: Make it possible to use pre-parsed info in git_difftree_body

gitweb: Make it possible to use pre-parsed info in git_difftree_body

Make it possible to use pre-parsed, or generated by hand, difftree
info in git_difftree_body, similarly to how was and is it done in
git_patchset_body.

Use just introduced feature in git_commitdiff to parse difftree info
(raw diff output) only once: difftree info is now parsed in
git_commitdiff directly, and parsed information is passed to both
git_difftree_body and git_patchset_body. (Till now only git_blobdiff
made use of git_patchset_body ability to use pre-parsed or hand
generated info.) Additionally this makes rename info for combined diff
with renames (or copies) calculated only once in git_difftree_body;
the $difftree is modified and git_patchset_body makes use of added
info.

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