path/filepath: handle ".." in normalizing a path on Windows

path/filepath: handle ".." in normalizing a path on Windows

Current code assumes there are not ".." in the Clean(path).
That's not true. Clean doesn't handle leading "..", so we need to stop
normalization if we see "..".

Fixes #16793

Change-Id: I0a7901bedac17f1210b134d593ebd9f5e8483775
Reviewed-on: https://go-review.googlesource.com/27410
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>