am: learn passing -b to mailinfo

am: learn passing -b to mailinfo

git-am could pass -k to mailinfo, but not -b.  Introduce an option
that does so.  We change the meaning of the 'keep' state file, but are
careful not to cause a problem unless you downgrade in the middle of
an 'am' run.

This uncovers a bug in mailinfo -b, hence the failing test.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>