format-patch: learn format.forceInBodyFrom configuration variable

format-patch: learn format.forceInBodyFrom configuration variable

As the need to use the "--force-in-body-from" option primarily is
tied to which mailing list the mails go to (and get their From:
address mangled), it is likely that a user who needs to use this
option once to interact with their upstream project needs to use it
for all patches they send out.

Add a configuration variable, suitable for setting in the local
configuration file per repository, for this.

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