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>

1 files changed, 2 insertions, 2 deletions

diff --git a/Documentation/gittutorial-2.txt b/Documentation/gittutorial-2.txt
index f1e4422..e00a4d2 100644
--- a/Documentation/gittutorial-2.txt
+++ b/Documentation/gittutorial-2.txt
@@ -34,12 +34,12 @@ $ echo 'hello world' > file.txt
 $ git add .
 $ git commit -a -m "initial commit"
 [master (root-commit) 54196cc] initial commit
- 1 files changed, 1 insertions(+), 0 deletions(-)
+ 1 file changed, 1 insertion(+)
  create mode 100644 file.txt
 $ echo 'hello world!' >file.txt
 $ git commit -a -m "add emphasis"
 [master c4d59f3] add emphasis
- 1 files changed, 1 insertions(+), 1 deletions(-)
+ 1 file changed, 1 insertion(+), 1 deletion(-)
 ------------------------------------------------
 
 What are the 7 digits of hex that git responded to the commit with?