Get(os.FileInfo) on Windows (7+) opens the file

[djherbis]

Get(os.FileInfo) opens the file on Win 7+ in order to obtain the ChangeTime via a file handle (which is not directly available on the Go struct). This is both expensive, and potentially problematic (my tests so far show that it doesn't seem to affect any of the file times, but I don't really want to do excessive actions behind the scenes).

I'm considering turning this off for Get and only having it active for Stat. I think this makes sense since Get should not have to Open the file, and Get should only make use of the data inside os.FileInfo.

I'm considering making this a version change as well.

Any suggestions? Complaints? Comments?

[djherbis]

Stat() now will open the file to get a handle and get ChangeTime.
Get() will only make use of the data already available in the FileInfo.Sys()
This means no ChangeTime from Get(), but much a faster call. [100x speedup]