difftool: use Git::* functions instead of passing around state

difftool: use Git::* functions instead of passing around state

Call Git::command() and friends directly wherever possible.
This makes it clear that these operations can be invoked directly
without needing to manage the current directory and related GIT_*
environment variables.

Eliminate find_repository() since we can now use wc_path() and
not worry about side-effects involving environment variables.

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