Save dep-tracking hash of commandline arguments in dep-graph file.

.. and use it to purge the incremental compilation cache if necessary.

src/test/incremental/commandline-args.rs

@@ -0,0 +1,30 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test that changing a tracked commandline argument invalidates
+// the cache while changing an untracked one doesn't.
+
+// revisions:rpass1 rpass2 rpass3
+
+#![feature(rustc_attrs)]
+
+#![rustc_partition_translated(module="commandline_args", cfg="rpass2")]
+#![rustc_partition_reused(module="commandline_args", cfg="rpass3")]
+
+// Between revisions 1 and 2, we are changing the debuginfo-level, which should
+// invalidate the cache. Between revisions 2 and 3, we are adding `--verbose`
+// which should have no effect on the cache:
+//[rpass1] compile-flags: -C debuginfo=0
+//[rpass2] compile-flags: -C debuginfo=2
+//[rpass3] compile-flags: -C debuginfo=2 --verbose
+
+pub fn main() {
+    println!("hello world");
+}