gitweb: Separate printing difftree in git_commit into git_difftree_body

gitweb: Separate printing difftree in git_commit into git_difftree_body

Separate printing difftree in git_commit into separate
git_difftree_body subroutine. Add support for "C" (copied) status. For
"M" and "C" add parameter 'fp' (filename parent) to the "diff" link;
currently not supported by git_blobdiff ("blobdiff" action).

Reindented, realigned, added comments.

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