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

When the array is empty `cmp` never gets set by the comparison
function. Initialize it so we return ENOTFOUND in those cases.

Remove the now-unnecessary entries vector.  Add `git_array_search`
to binary search through an array to accomplish this.

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.

Use `size_t` to hold the size of arrays to ease overflow checking,
lest we check for overflow of a `size_t` then promptly truncate
by packing the length into a smaller type.

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 works around strict aliasing rules letting some versions of
GCC (particularly on RHEL 6) thinking that they can skip updating the
size of the array when calculating the next element's offset.

* Make GIT_INLINE an internal definition so it cannot be used in
  public headers
* Fix language in CONTRIBUTING
* Make index caps API use signed instead of unsigned values