Add --summary to git-format-patch by default

This adds --summary output in addition to the --stat to the
output from git-format-patch by default.

I think additions, removals and filemode changes are rare but
notable events and always showing it makes sense.

Signed-off-by: Junio C Hamano <junkio@cox.net>

t/t4013/diff.format-patch_--stdout_initial..master^

@@ -10,6 +10,7 @@ This is the second commit.
  file0   |    3 +++
  file2   |    3 ---
  3 files changed, 5 insertions(+), 3 deletions(-)
+ delete mode 100644 file2
 
 diff --git a/dir/sub b/dir/sub
 index 35d242b..8422d40 100644
@@ -53,6 +54,7 @@ Subject: [PATCH] Third
  dir/sub |    2 ++
  file1   |    3 +++
  2 files changed, 5 insertions(+), 0 deletions(-)
+ create mode 100644 file1
 
 diff --git a/dir/sub b/dir/sub
 index 8422d40..cead32e 100644