test-path-utils: handle const parameter of basename and dirname

test-path-utils: handle const parameter of basename and dirname

The parameter to basename(3) and dirname(3) traditionally had the type
"char *", but on OpenBSD it's been "const char *" for years.  That
causes (at least) Clang to throw an incompatible-pointer-types warning
for test-path-utils, where we try to pass around pointers to these
functions.

Avoid this warning (which is fatal in DEVELOPER mode) by ignoring the
promise of OpenBSD's implementations to keep input strings unmodified
and enclosing them in POSIX-compatible wrappers.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>