range-diff: drop const to fix strstr() warnings

range-diff: drop const to fix strstr() warnings

This is another case where we implicitly drop the "const" from a pointer
by feeding it to strstr() and assigning the result to a non-const
pointer. This is OK in practice, since the const pointer originally
comes from a writable source (a strbuf), but C23 libc implementations
have started to complain about it.

We do write to the output pointer, so it needs to remain non-const. We
can just switch the input pointer to also be non-const in this case.  By
itself that would run into problems with calls to skip_prefix(), but
since that function has now been taught to match in/out constness
automatically, it just works without us doing anything further.

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