diff --stat: do not count "unmerged" entries

diff --stat: do not count "unmerged" entries

Even though we show a separate *UNMERGED* entry in the patch and
diffstat output (or in the --raw format, for that matter) in
addition to and separately from the diff against the specified stage
(defaulting to #2) for unmerged paths, they should not be counted in
the total number of files affected---that would lead to counting the
same path twice.

The separation done by the previous step makes this fix simple and
straightforward.  Among the filepairs in diff_queue, paths that
weren't modified, and the extra "unmerged" entries do not count as
total number of files.

Signed-off-by: Junio C Hamano <gitster@pobox.com>