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/diff.c

@@ -475,6 +475,7 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 
 	/* Set up defaults that will apply to both no-index and regular diffs. */
 	rev.diffopt.stat_width = -1;
+	rev.diffopt.stat_name_width = -1;
 	rev.diffopt.stat_graph_width = -1;
 	rev.diffopt.flags.allow_external = 1;
 	rev.diffopt.flags.allow_textconv = 1;