diff --stat: tests for long filenames and big change counts

diff --stat: tests for long filenames and big change counts

In preparation for updates to the "diff --stat" that updates the logic
to split the allotted columns into the name part and the graph part to
make the output more readable, add a handful of tests to document the
corner case behaviour in which long filenames and big changes are shown.

When a pathname is so long that it cannot fit on the column, the current
code truncates it to make sure that the graph part has enough room to show
a meaningful graph.  If the actual change is small (e.g. only one line
changed), this results in the final output that is shorter than the width
we aim for.

Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>