largefiles: use the core file copy logic to validate the destination path The destination is validated by pathutil.canonpath() for illegal components, and that it is in the repository. The logic for creating the standin directory tree was calling this before cmdutil.copy(), but without the destination file name component. The cmdutil.copy() logic also calls pathutil.canonpath(), but with the file name component. By always calling the core logic first, the error message is always consistent. Specifically, the old behavior for these tests was to say '.hg' contains an illegal component, and '..' is not under root. A user wasn't likely to notice the discrepancy, but this eliminates a needless difference when running the test suite with --config extensions.largefiles=.