difftool: fix argument handling in subdirs

difftool: fix argument handling in subdirs

When in a subdirectory of a repository, path arguments should be
interpreted relative to the current directory not the root of the
working tree.

The Git::repository object passed into setup_dir_diff() is configured to
handle this correctly but we create a new Git::repository here without
setting the WorkingSubdir argument.  By simply using the existing
repository, path arguments are handled relative to the current
directory.

Reported-by: Bernhard Kirchen <bernhard.kirchen@rwth-aachen.de>
Signed-off-by: John Keeping <john@keeping.me.uk>
Acked-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>