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>

diff.h

@@ -573,6 +573,7 @@ int git_config_rename(const char *var, const char *value);
 
 #define DIFF_PICKAXE_IGNORE_CASE	32
 
+void init_diffstat_widths(struct diff_options *);
 void diffcore_std(struct diff_options *);
 void diffcore_fix_diff_index(void);