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>

t/t4013/diff.diff-tree_-c_--stat_master

@@ -2,5 +2,5 @@ $ git diff-tree -c --stat master
 59d314ad6f356dd08601a4cd5e530381da3e3c64
  dir/sub |    2 ++
  file0   |    3 +++
- 2 files changed, 5 insertions(+), 0 deletions(-)
+ 2 files changed, 5 insertions(+)
 $