diff --stat: add config option to limit filename width

Add new configuration option diff.statNameWidth=<width> that is equivalent
to the command-line option --stat-name-width=<width>, but it is ignored
by format-patch.  This follows the logic established by the already
existing configuration option diff.statGraphWidth=<width>.

Limiting the widths of names and graphs in the --stat output makes sense
for interactive work on wide terminals with many columns, hence the support
for these configuration options.  They don't affect format-patch because
it already adheres to the traditional 80-column standard.

Update the documentation and add more tests to cover new configuration
option diff.statNameWidth=<width>.  While there, perform a few minor code
and whitespace cleanups here and there, as spotted.

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

builtin/rebase.c

@@ -1804,6 +1804,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		/* We want color (if set), but no pager */
 		repo_diff_setup(the_repository, &opts);
 		opts.stat_width = -1; /* use full terminal width */
+		opts.stat_name_width = -1; /* respect statNameWidth config */
 		opts.stat_graph_width = -1; /* respect statGraphWidth config */
 		opts.output_format |=
 			DIFF_FORMAT_SUMMARY | DIFF_FORMAT_DIFFSTAT;