Introduce a stricter debugging allocator for testing #6811
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Instead of tweaking the `stdalloc` allocator when `GIT_DEBUG_STRICT_ALLOC` is defined, actually create a debugging allocator. This allows us to ensure that we are strict about things like not expecting `malloc(0)` to do something useful, but we can also introduce an excessively pedantic `realloc` implementation that _always_ creates a new buffer, throws away its original `ptr`, and overwrites the data that's there with garbage. This may be helpful to identify places that make assumptions about realloc.
realpath(3) _may_ allocate strings (if the second param is NULL) using the system allocator. However, callers need an assurance that they can free memory using git__free. If we made realpath do an allocation, then make sure that we strdup it into our allocator's memory. More importantly, avoid this behavior by always providing a buffer to p_realpath invocations.
Instead of setting the allocator to stdalloc, just pass `NULL`, in case we're running with the debug allocator.
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Introduce a new debugging allocator that:
mallocis not the same pointer that the underlying systemmallocused, and is therefore incompatible with the systemfree. This helps catch inconsistent use of system malloc and the libgit2-configured malloc.reallocto always reallocate (it will explicitlymalloc,memcpy, andfreethe previous pointer)Also, update some inconsistent use of allocators caught by 1.