diff --stat: set the width defaults in a helper function

diff --stat: set the width defaults in a helper function

Extract the commonly used initialization of the --stat-width=<width>,
--stat-name-width=<width> and --stat-graph-with=<width> parameters to their
internal default values into a helper function, to avoid repeating the same
initialization code in a few places.

Add a couple of tests to additionally cover existing configuration options
diff.statNameWidth=<width> and diff.statGraphWidth=<width> when used by
git-merge to generate --stat outputs.  This closes the gap that existed
previously in the --stat tests, and reduces the chances for having any
regressions introduced by this commit.

While there, perform a small bunch of minor wording tweaks in the improved
unit test, to improve its test-level consistency a bit.

Signed-off-by: Dragan Simic <dsimic@manjaro.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>