merge/pull: extend merge.stat configuration variable to cover --compact-summary

merge/pull: extend merge.stat configuration variable to cover --compact-summary

Existing `merge.stat` configuration variable is a Boolean that
defaults to `true` to control `git merge --[no-]stat` behaviour.

Extend it to be "Boolean or text", that takes false, true, or
"compact", with the last one triggering the --compact-summary option
introduced earlier.  Any other values are taken as the same as true,
instead of signaling an error---it is not a grave enough offence to
stop their merge.

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