Instead of tweaking the `stdalloc` allocator when`GIT_DEBUG_STRICT_ALLOC` is defined, actually create a debuggingallocator. This allows us to ensure that we are strict about things likenot expecting `malloc(0)` to do something useful, but we can alsointroduce an excessively pedantic `realloc` implementation that _always_creates a new buffer, throws away its original `ptr`, and overwrites thedata that's there with garbage. This may be helpful to identify placesthat make assumptions about realloc.

realpath(3) _may_ allocate strings (if the second param is NULL) usingthe system allocator. However, callers need an assurance that they canfree memory using git__free. If we made realpath do an allocation, thenmake sure that we strdup it into our allocator's memory.More importantly, avoid this behavior by always providing a buffer top_realpath invocations.

Instead of setting the allocator to stdalloc, just pass `NULL`, in casewe're running with the debug allocator.