diff: add --compact-summary

Certain information is currently shown with --summary, but when used
in combination with --stat it's a bit hard to read since info of the
same file is in two places (--stat and --summary).

On top of that, commits that add or remove files double the number of
display lines, which could be a lot if you add or remove a lot of
files.

--compact-summary embeds most of --summary back in --stat in the
little space between the file name part and the graph line, e.g. with
commit 0433d533f1:

   Documentation/merge-config.txt         |  4 +
   builtin/merge.c                        |  2 +
   ...-pull-verify-signatures.sh (new +x) | 81 ++++++++++++++
   t/t7612-merge-verify-signatures.sh     | 45 ++++++++
   4 files changed, 132 insertions(+)

It helps both condensing information and saving some text
space. What's new in diffstat is:

- A new 0644 file is shown as (new)
- A new 0755 file is shown as (new +x)
- A new symlink is shown as (new +l)
- A deleted file is shown as (gone)
- A mode change adding executable bit is shown as (mode +x)
- A mode change removing it is shown as (mode -x)

Note that --compact-summary does not contain all the information
--summary provides. Rewrite percentage is not shown but it could be
added later, like R50% or C20%.

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

Documentation/diff-options.txt

@@ -128,6 +128,14 @@ have to use `--diff-algorithm=default` option.
 These parameters can also be set individually with `--stat-width=<width>`,
 `--stat-name-width=<name-width>` and `--stat-count=<count>`.
 
+--compact-summary::
+	Output a condensed summary of extended header information such
+	as file creations or deletions ("new" or "gone", optionally "+l"
+	if it's a symlink) and mode changes ("+x" or "-x" for adding
+	or removing executable bit respectively) in diffstat. The
+	information is put betwen the filename part and the graph
+	part. Implies `--stat`.
+
 --numstat::
 	Similar to `--stat`, but shows number of added and
 	deleted lines in decimal notation and pathname without