branch: add branch.<name>.deleteMerged opt-out

branch: add branch.<name>.deleteMerged opt-out

Setting branch.<name>.deleteMerged=false exempts that branch from
"git branch --delete-merged", which is useful for a topic you want
to keep developing after an early round of it has been merged
upstream. Unless --quiet is given, each skip is reported so the
user knows why their topic was kept.

Explicit deletion with "git branch -d" still uses the normal merge
check and ignores this setting.

Signed-off-by: Harald Nordgren <haraldnordgren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>