* path-concat.c: Don't include assert.h.

* path-concat.c: Don't include assert.h.
(path_concat): Remove assertion that would have triggered
for ABASE starting with more than one slash.
Reported by Andreas Schwab.

* path-concat.c (path_concat): Set *BASE_IN_RESULT
properly when ABASE is an absolute file name.
Correct the description of this function.
Include <assert.h>.
Add an assertion and a test driver.
This fixes a bug introduced on 2004-07-02.
Andreas Schwab reported the resulting failure of cp --parents:
http://lists.gnu.org/archive/html/bug-coreutils/2005-01/msg00130.html