git-am: add --message-id/--no-message-id

Parse the option and pass it directly to git-mailinfo.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-am.txt

@@ -57,6 +57,17 @@ OPTIONS
 --no-scissors::
 	Ignore scissors lines (see linkgit:git-mailinfo[1]).
 
+-m::
+--message-id::
+	Pass the `-m` flag to 'git mailinfo' (see linkgit:git-mailinfo[1]),
+	so that the Message-ID header is added to the commit message.
+	The `am.messageid` configuration variable can be used to specify
+	the default behaviour.
+
+--no-message-id::
+	Do not add the Message-ID header to the commit message.
+	`no-message-id` is useful to override `am.messageid`.
+
 -q::
 --quiet::
 	Be quiet. Only print error messages.