mergetool: fix running in subdir when rerere enabled

mergetool: fix running in subdir when rerere enabled

"git mergetool" (without any pathspec on the command line) that is
not run from the top-level of the working tree no longer works in
Git v2.11, failing to get the list of unmerged paths from the output
of "git rerere remaining".  This regression was introduced by
57937f70a0 ("mergetool: honor diff.orderFile", 2016-10-07).

This is because the pathnames output by the 'git rerere remaining'
command are relative to the top-level directory but the 'git diff
--name-only' command expects its pathname arguments to be relative
to the current working directory.  To make everything consistent,
cd_to_toplevel before running 'git diff --name-only' and adjust any
relative pathnames.

Signed-off-by: Richard Hansen <hansenr@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>