Use correct grammar in diffstat summary line

"git diff --stat" and "git apply --stat" now learn to print the line
"%d files changed, %d insertions(+), %d deletions(-)" in singular form
whenever applicable. "0 insertions" and "0 deletions" are also omitted
unless they are both zero.

This matches how versions of "diffstat" that are not prehistoric produced
their output, and also makes this line translatable.

[jc: with help from Thomas Dickey in archaeology of "diffstat"]
[jc: squashed Jonathan's updates to illustrations in tutorials and a test]

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.h

@@ -324,4 +324,7 @@ extern struct userdiff_driver *get_textconv(struct diff_filespec *one);
 
 extern int parse_rename_score(const char **cp_p);
 
+extern int print_stat_summary(FILE *fp, int files,
+			      int insertions, int deletions);
+
 #endif /* DIFF_H */