branch: convert delete_branches() to a flags argument

branch: convert delete_branches() to a flags argument

delete_branches() takes separate force and quiet parameters, while
check_branch_commit() takes force. The next commits would grow this
collection further. Replace them with a single unsigned flags argument
and an enum.

Test the FORCE and QUIET bits directly from flags at each use site so
that mutating or forwarding flags cannot leave cached values stale.

No change in behavior.

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