Use correct grammar in diffstat summary linend/diffstat-gramnum

"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, 3 insertions, 3 deletions

diff --git a/t/t7602-merge-octopus-many.sh b/t/t7602-merge-octopus-many.sh
index 61f36baa1f..5783ebf3ab 100755
--- a/t/t7602-merge-octopus-many.sh
+++ b/t/t7602-merge-octopus-many.sh
@@ -57,7 +57,7 @@ Merge made by the 'octopus' strategy.
  c2.c |    1 +
  c3.c |    1 +
  c4.c |    1 +
- 3 files changed, 3 insertions(+), 0 deletions(-)
+ 3 files changed, 3 insertions(+)
  create mode 100644 c2.c
  create mode 100644 c3.c
  create mode 100644 c4.c
@@ -74,7 +74,7 @@ Already up-to-date with c4
 Trying simple merge with c5
 Merge made by the 'octopus' strategy.
  c5.c |    1 +
- 1 files changed, 1 insertions(+), 0 deletions(-)
+ 1 file changed, 1 insertion(+)
  create mode 100644 c5.c
 EOF
 
@@ -89,7 +89,7 @@ Trying simple merge with c2
 Merge made by the 'octopus' strategy.
  c1.c |    1 +
  c2.c |    1 +
- 2 files changed, 2 insertions(+), 0 deletions(-)
+ 2 files changed, 2 insertions(+)
  create mode 100644 c1.c
  create mode 100644 c2.c
 EOF