bisect: use string arguments to feed internal diff-tree

bisect: use string arguments to feed internal diff-tree

Commit e22278c0a0 (bisect: display first bad commit without forking a
new process, 2009-05-28) converted our external call to diff-tree to
an internal use of the log_tree_commit(). But rather than individually
setting options in the rev_info struct (and explaining in comments how
they map to command-line options), we can just pass the command-line
options to setup_revisions().

This is shorter, easier to change, and less likely to break if
revision.c internals change.

Note that we unconditionally set the output format to "raw". The
conditional in the original code didn't actually do anything useful,
since nobody had an opportunity to set the format to anything.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>