difftool: Handle compare() returning -1

difftool: Handle compare() returning -1

Keep the temporary directory around when compare()
cannot read its input files, which is indicated by -1.

Defer tempdir creation to allow an early exit in setup_dir_diff().
Wrap the rest of the entry points in an exit_cleanup() function
to handle removing temporary files and error reporting.

Print the temporary files' location so that the user can
recover them.

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