manifest: add basic path checks for <copyfile> & <linkfile>

Reject paths in <copyfile> & <linkfile> that point outside of their
respective scopes.  This validates paths while parsing the manifest
as this should be quick & cheap: we don't access the filesystem as
this code runs before we've synced.

Bug: https://crbug.com/gerrit/11218
Change-Id: I8e17bb91f3f5b905a9d76391b29fbab4cb77aa58
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/232932


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Michael Mortensen <mmortensen@google.com>