gitweb: Add "tree_blame" view (WIP)

gitweb: Add "tree_blame" view (WIP)

Adds git_tree_blame subroutine, for the "tree_blame" view, similar to
the ViewVC output.  It means having the commit, date and author of
last change to the file (blob) or directory (tree) in given tree in
addition to the default "tree" view info.

The git_tree_blame subroutine contains 3 implementations: one using
git-diff-tree to detect when file was changed (works only on blobs,
and not on trees/directories), one comparing output of git-ls-tree to
check what changed between revisions, and one (default used) looking
at two first commits in the blob or tree history.

While at it, add 'year' field to the parse_date output; add "blame"
link to the list of links for tree entries in directory listing in
git_print_tree_entry; add support for 'order' parameter to href().

Signed-off-by: Jakub Narebski <jnareb@gmail.com>