cpplint: Fix --root for non-subdirectories

Using cpplint.py --root with directories at a more outer level
than the repository would not work.

For example given

  /a/b/c/.git/filename.cpp

Trying to use --root=/a/b would get ignored, and it would
ask for a headerguard of FILENAME_H_ instead of C_FILENAME_H_
as expected.

Now --root will always use the "full name" and then strip off --root
prefix, giving us the desired effect.