format-patch: allow forcing the use of in-body From: header

format-patch: allow forcing the use of in-body From: header

Users may be authoring and committing their commits under the same
e-mail address they use to send their patches from, in which case
they shouldn't need to use the in-body From: line in their outgoing
e-mails.  At the receiving end, "git am" will use the address on the
"From:" header of the incoming e-mail and all should be well.

Some mailing lists, however, mangle the From: address from what the
original sender had; in such a situation, the user may want to add
the in-body "From:" header even for their own patches.

"git format-patch --[no-]force-in-body-from" was invented for such
users.

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