The code reversing a vector initially determines the rear-pointer by
simply subtracting 1 from the vector's length. Obviously, this fails if
the vector is empty, in which case we have an integer overflow.

Fix the issue by returning early if the vector is empty.

This is a convenience function to reverse the contents of a vector and a pqueue
in-place.

The pqueue function is useful in the case where we're treating it as a
LIFO queue.

The beginnings of patch application from an existing (diff-created)
git_patch object: applies the hunks of a git_patch to a buffer.

Make our overflow checking look more like gcc and clang's, so that
we can substitute it out with the compiler instrinsics on platforms
that support it.  This means dropping the ability to pass `NULL` as
an out parameter.

As a result, the macros also get updated to reflect this as well.

Introduce git__reallocarray that checks the product of the number
of elements and element size for overflow before allocation.  Also
introduce git__mallocarray that behaves like calloc, but without the
`c`.  (It does not zero memory, for those truly worried about every
cycle.)

Introduce some helper macros to test integer overflow from arithmetic
and set error message appropriately.

This makes the index iterator honor the GIT_ITERATOR_IGNORE_CASE
and GIT_ITERATOR_DONT_IGNORE_CASE flags without modifying the
index data itself.  To take advantage of this, I had to export a
number of the internal index entry comparison functions.  I also
wrote some new tests to exercise the capability.