git-send-email: allow an email alias for --from

Signed-off-by: Michael Hendricks <michael@ndrix.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -254,6 +254,8 @@ if (@alias_files and $aliasfiletype and defined $parse_alias{$aliasfiletype}) {
 	}
 }
 
+($from) = expand_aliases($from) if defined $from;
+
 my $prompting = 0;
 if (!defined $from) {
 	$from = $author || $committer;