init_revisions() initializes the rev_info struct to default
values, and setup_revisions() parses any command-line
arguments and finalizes the struct.

In e22278c (bisect: display first bad commit without forking
a new process, 2009-05-28), a show_diff_tree() was added
that calls the former but not the latter. It doesn't have
any arguments to parse, but it still should do the
finalizing step.

This may have caused other minor bugs over the years, but it
became much more prominent after fe37a9c (pretty: allow
tweaking tabwidth in --expand-tabs, 2016-03-29). That leaves
the expected tab width as "-1", rather than the true default
of "8". When we see a commit with tabs to be expanded, we
end up trying to add (size_t)-1 spaces to a strbuf, which
complains about the integer overflow.

The fix is easy: just call setup_revisions() with no
arguments.

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