t7800-difftool: Set a bogus tool for use by tests

t7800-difftool: Set a bogus tool for use by tests

If a difftool test has an error then running the git test suite
may end up invoking a GUI diff tool.  We now guard against this
by setting a difftool.bogus-tool.cmd variable.

The tests already used --tool=bogus-tool in various places so
this is simply ensuring that nothing ever falls back and
finds a real diff tool.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>