Fix regex escape issue when using --root on Windows

The code was using directory separator in the regex itself,
which means on Windows it was leaving unescaped backslash.

Added explicit escape for such cases.

cpplint/cpplint.py

@@ -1746,7 +1746,12 @@ def GetHeaderGuardCPPVariable(filename):
   fileinfo = FileInfo(filename)
   file_path_from_root = fileinfo.RepositoryName()
   if _root:
-    file_path_from_root = re.sub('^' + _root + os.sep, '', file_path_from_root)
+    suffix = os.sep
+    # On Windows using directory separator will leave us with
+    # "bogus escape error" unless we properly escape regex.
+    if suffix == '\\':
+      suffix += '\\'
+    file_path_from_root = re.sub('^' + _root + suffix, '', file_path_from_root)
   return re.sub(r'[^a-zA-Z0-9]', '_', file_path_from_root).upper() + '_'