Add flag to handle only *newer* files not *older*

[alerque]

The primary concern GWT set out to fix was actions like git checkout marking files as newer than their actual last modification times. In the process we ended up resetting all files to their Git times.

One hack around for some build systems is to touch a file with a very old date to make sure it does not trigger builds. GWT currency clobbers these hacks.

We could add a flag to not reset files that are older on disk than their respective last commit times.