compat/basename.c: provide a dirname() compatibility function

compat/basename.c: provide a dirname() compatibility function

When there is no `libgen.h` to our disposal, we miss the `dirname()`
function.  Earlier we added basename() compatibility function for
the same reason at e1c06886 (compat: add a basename() compatibility
function, 2009-05-31).

So far, we only had one user of that function: credential-cache--daemon
(which was only compiled when Unix sockets are available, anyway). But
now we also have `builtin/am.c` as user, so we need it.

Since `dirname()` is a sibling of `basename()`, we simply put our very
own `gitdirname()` implementation next to `gitbasename()` and use it
if `NO_LIBGEN_H` has been set.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>