Test and fix normalize_path_copy()

Test and fix normalize_path_copy()

This changes the test-path-utils utility to invoke normalize_path_copy()
instead of normalize_absolute_path() because the latter is about to be
removed.

The test cases in t0060 are adjusted in two regards:

- normalize_path_copy() more often leaves a trailing slash in the result.
  This has no negative side effects because the new user of this function,
  longest_ancester_length(), already accounts for this behavior.

- The function can fail.

The tests uncover a flaw in normalize_path_copy(): If there are
sufficiently many '..' path components so that the root is reached, such as
in "/d1/s1/../../d2", then the leading slash was lost. This manifested
itself that (assuming there is a repository at /tmp/foo)

  $ git add /d1/../tmp/foo/some-file

reported 'pathspec is outside repository'. This is now fixed.

Moreover, the test case descriptions of t0060 now include the test data and
expected outcome.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>