Instead of creating symlinks on Windows, the script is now:
* creating a junction for directories
* copying individual files.

This makes 'gclient sync' and 'gclient runhooks' no longer
require administrator's privileges.
If the script is run with administrator's privileges, a
warning will be printed, informing the user that it's not recommended.

Also clean up a few old documentation references to the
Chromium SVN->Git transition.

BUG=libyuv:584,webrtc:4911
TESTED=Running the script with+without administrator's privileges.
I also tested the case of this change being rolled back, in which
case I verified that the copied files are still being deleted using
the same cleanup code path as the previous symlinks.
NOTRY=True
R=braveyao@chromium.org

Review URL: https://codereview.chromium.org/2359383002 .