find_last_dir_sep(): convert inline function to macro

find_last_dir_sep(): convert inline function to macro

The find_last_dir_sep() function is implemented as an inline function
which takes in a "const char *" and returns a "char *" via strrchr().
That means that just like strrchr(), it quietly removes the const from
our pointer, which could lead to accidentally writing to the resulting
string.

But C23 versions of libc (including recent glibc) annotate strrchr()
such that the compiler can detect when const is implicitly lost, and it
now complains about the call in this inline function.

We can't just switch the return type of the function to "const char *",
though. Some callers really do want a non-const string to be returned
(and are OK because they are feeding a non-const string into the
function).

The most general solution is for us to annotate find_last_dir_sep() in
the same way that is done for strrchr(). But doing so relies on using
C23 generics, which we do not otherwise require.

Since this inline function is wrapping a single call to strrchr(), we
can take a shortcut. If we implement it as a macro, then the original
type information is still available to strrchr(), and it does the check
for us.

Note that this is just one implementation of find_last_dir_sep(). There
is an alternate implementation in compat/win32/path-utils.h. It doesn't
suffer from the same warning, as it does not use strrchr() and just
casts away const explicitly. That's not ideal, and eventually we may
want to conditionally teach it the same C23 generic trick that strrchr()
uses.  But it has been that way forever, and our goal here is just
quieting new warnings, not improving const-checking.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>