send-email: make message-id generation a bit more robust

Earlier code took Unix time and appended a few random digits.
If you are firing off many messages within a second, you could
issue the same id to different messages, which is a no-no.  If
you send out 31 messages within a single second, with random
integer taken out of rand(4200), you have about 10% chance of
producing the same message ID.

This fixes the problem by uses a prefix string which is
constant-per-invocation (time and pid), with a serial number for
each message generated by the process appended at the end.

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

git-send-email.perl

@@ -437,10 +437,17 @@ sub extract_valid_address {
 
 # We'll setup a template for the message id, using the "from" address:
 
+my ($message_id_stamp, $message_id_serial);
 sub make_message_id
 {
-	my $date = time;
-	my $pseudo_rand = int (rand(4200));
+	my $uniq;
+	if (!defined $message_id_stamp) {
+		$message_id_stamp = sprintf("%s-%s", time, $$);
+		$message_id_serial = 0;
+	}
+	$message_id_serial++;
+	$uniq = "$message_id_stamp-$message_id_serial";
+
 	my $du_part;
 	for ($sender, $repocommitter, $repoauthor) {
 		$du_part = extract_valid_address(sanitize_address($_));
@@ -450,8 +457,8 @@ sub make_message_id
 		use Sys::Hostname qw();
 		$du_part = 'user@' . Sys::Hostname::hostname();
 	}
-	my $message_id_template = "<%s-git-send-email-$du_part>";
-	$message_id = sprintf $message_id_template, "$date$pseudo_rand";
+	my $message_id_template = "<%s-git-send-email-%s>";
+	$message_id = sprintf($message_id_template, $uniq, $du_part);
 	#print "new message id = $message_id\n"; # Was useful for debugging
 }