Skip timestamp differences for diff --no-index

We display empty diffs for files whose timestamps have changed.
Usually, refreshing the index makes those empty diffs go away.
However, when not using the index they are not very useful and
there is no option to suppress them.

This forces on the skip_stat_unmatch option for diff --no-index,
suppressing any empty diffs. This option is also used for diffs
against the index when "diff.autorefreshindex" is set, but that
option does not apply to diff --no-index.

Signed-off-by: Michael Spang <mspang@uwaterloo.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4013/diff.diff_--no-index_dir_dir3

@@ -0,0 +1,2 @@
+$ git diff --no-index dir dir3
+$