diff: introduce --stat-lines to limit the stat lines

Often one is interested in the full --stat output only for commits which
change a few files, but not others, because larger restructuring gives a
--stat which fills a few screens.

Introduce a new option --stat-count=<count> which limits the --stat output
to the first <count> lines, followed by a "..." line. It can
also be given as the third parameter in
--stat=<width>,<name-width>,<count>.

Also, the unstuck form is supported analogous to the other two stat
parameters.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.h

@@ -124,6 +124,7 @@ struct diff_options {
 
 	int stat_width;
 	int stat_name_width;
+	int stat_count;
 	const char *word_regex;
 	enum diff_words_type word_diff;