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>

revision.h

@@ -221,6 +221,7 @@ struct rev_info {
 			missing_newline:1,
 			date_mode_explicit:1,
 			preserve_subject:1,
+			force_in_body_from:1,
 			encode_email_headers:1,
 			include_header:1;
 	unsigned int	disable_stdin:1;