git-send-email: Don't set author_not_sender from Cc: lines

When an mbox-style patch contains a Cc: line in the header,
git-send-email will check the address against the sender specified
on the command line. If they don't match, sender_not_author will
be set to the address obtained from the Cc line.

When this happens, git-send-email inserts a From: line at the
beginning of the message body with the address obtained from the
Cc line in the header, and the sender might be accused of forging
patch authors.

This patch fixes this by only updating sender_not_author when
processing From: lines, not when processing Cc: lines.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-send-email.perl

@@ -510,7 +510,7 @@ sub send_message
 					if ($2 eq $from) {
 						next if ($suppress_from);
 					}
-					else {
+					elsif ($1 eq 'From') {
 						$author_not_sender = $2;
 					}
 					printf("(mbox) Adding cc: %s from line '%s'\n",