Make mytasks script take a origin to diff against

Change-Id: I8640b0852e96b21518fde36d0e0eb24ae473b6f9
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>

scripts/mytasks.pl

@@ -88,7 +88,8 @@ sub getDiffOrigin {
    return "$remoteRepo/$remoteBranch";
 }
 
-my $origin = getDiffOrigin;
+my $origin = shift;
+$origin = getDiffOrigin unless $origin;
 print "# running: git diff $origin ...\n";
 
 open(PIPE, "git diff $origin|");