gitweb: Add combined diff support to git_difftree_body

gitweb: Add combined diff support to git_difftree_body

You have to pass all parents as final parameters of git_difftree_body
subroutine; the number of parents of a diff must be equal to the
number derived from parsing git-diff-tree output, raw combined diff
for git_difftree_body to display combined diff correctly (but it is
not checked).

Currently the possibility of displaying diffree of combined diff is
not used in gitweb code; git_difftree_body is always caled for
ordinary diff, and with only one parent.

Description of output for combined diff:
----------------------------------------

The difftree table for combined diff starts with a cell with pathname
of changed blob (changed file), which if possible is hidden link
(class="list") to the 'blob' view of final version (if it exists),
like for difftree for ordinary diff. If file was deleted in the final
commit then filename is not hyperlinked.

There is no cell with single file status (new, deleted, mode change,
rename), as for combined diff as there is no single status: different
parents might have different status.

If git_difftree_body was called from git_commitdiff (for 'commitdiff'
action) there is inner link to anchor to appropriate fragment (patch)
in patchset body; the "patch" link does not replace "diff" link like
for ordinary diff.

Each of "diff" links is in separate cell, contrary to output for
ordinary diff in which all links are (at least for now) in a single
cell.

For each parent, if file was not present we leave cell empty. If file
was deleted in the result, we provide link to 'blob' view. Otherwise
we provide link to 'commitdiff' view, even if patch (diff) consist
only of extended diff header, and contents is not changed (pure
rename, pure mode change). The only difference is that link to
"blobdiff" view with no contents change is with 'nochange' class.

At last, there is provided link to current version of file as "blob"
link, if the file was not deleted in the result, and lik to history of
a file, if there exists one. (The link to file history might be
confused, at least for now, by renames.)

Note that git-diff-tree raw output dor combined diff does not provide
filename before change for renames and copies; we use
git_get_path_by_hash to get "src" filename for renames (this means
additional call to git-ls-tree for a _whole_ tree).

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