Never overwrite destination paths.

This is the minimum change that lets us skip updating files in place in
the destination bucket. A follow up CL will clean up the logic related
to figuring out if a file has changed so that it can be updated.

BUG=chromium:703244
TEST=$ python
     >> import mirror
     >> mirror.Mirror(
     .. 'gs://chromeos-mirror/gentoo/distfiles/libseccomp-2.3.1.tar.gz',
     .. 'gs://chromeos-mirror/gentoo/distfiles/libseccomp-2.3.1.tar.gz',
     .. lambda x: 'blah'
     .. )

Change-Id: I63622cf9c0c099ea705c895100bcfe9eabf05105
Reviewed-on: https://chromium-review.googlesource.com/483863
Commit-Ready: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
1 file changed