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>

error.py

@@ -22,6 +22,10 @@ class ManifestInvalidRevisionError(Exception):
   """The revision value in a project is incorrect.
   """
 
+class ManifestInvalidPathError(Exception):
+  """A path used in <copyfile> or <linkfile> is incorrect.
+  """
+
 class NoManifestException(Exception):
   """The required manifest does not exist.
   """