manifest_xml: change .repo/manifest.xml to a plain file

Changing this to a file instead of using a symlink serves two purposes:
* We can insert some comments & doc links to help users learn what this
  is for, discover relevant documentation, and to discourage them from
  modifying things.
* Windows requires Administrator access to use symlinks.  With this
  last change, Windows users can get repo client checkouts with the new
  --worktree option and not need symlinks anywhere at all.  Which means
  they no longer need to be an Administrator in order to `repo sync`.

Change-Id: I9bc46824fd8d4b0f446ba84bd764994ca1e597e2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256313
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>