Our hand-rolled fallback sorting function `git__insertsort_r` does an
in-place sort of the given array. As elements may not necessarily be
pointers, it needs a way of swapping two values of arbitrary size, which
is currently implemented by allocating a temporary buffer of the
element's size. This is problematic, though, as the emulated `qsort`
interface doesn't provide any return values and thus cannot signal an
error if allocation of that temporary buffer has failed.

Convert the function to swap via a temporary buffer allocated on the
stack. Like this, it can `memcpy` contents of both elements in small
batches without requiring a heap allocation. The buffer size has been
chosen such that in most cases, a single iteration of copying will
suffice. Most importantly, it can fully contain `git_oid` structures and
pointers.

Add a bunch of tests for the `git__qsort_r` interface to verify nothing
breaks. Furthermore, this removes the declaration of `git__insertsort_r`
and makes it static as it is not used anywhere else.