merge/pull: add the "--compact-summary" option

merge/pull: add the "--compact-summary" option

"git merge" and "git pull" shows "git diff --stat --summary @{1}"
when they finish to indicate the extent of the changes brought into
the history by default.  While it gives a good overview, it becomes
annoying when there are very many created or deleted paths.

Introduce "--compact-summary" option to these two commands that
tells it to instead show "git diff --compact-summary @{1}", which
gives the same information in a lot more compact form in such a
situation.

Signed-off-by: Junio C Hamano <gitster@pobox.com>