Make difftool chdir to the top-level of the repository as soon as it can
so that we can simplify how paths are handled. Replace construction of
absolute paths via string concatenation with relative paths wherever
possible. The bulk of the code no longer needs to use absolute paths.
Signed-off-by: David Aguilar <davvid@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>