When writing an object, we calculate its OID and see if it exists in the
object database.  If it does, we need to freshen the file that contains
it.

I don't quite recall what we do in the other places where we use this,
but we should pass this payload to the callbacks.

Since the `apply` callback can defer, the `check` callback is not
necessary.  Removing the `check` callback further makes the `payload`
unnecessary along with the `cleanup` callback.

Consumers can now register custom merged drivers with
`git_merge_driver_register`.  This allows consumers to support the
merge drivers, as configured in `.gitattributes`.  Consumers will be
asked to perform the file-level merge when a custom driver is
configured.

The overflow check in `read_reuc` tries to verify if the
`git__strtol32` parses an integer bigger than UINT_MAX. The `tmp`
variable is casted to an unsigned int for this and then checked
for being greater than UINT_MAX, which obviously can never be
true.

Fix this by instead fixing the `mode` field's size in `struct
git_index_reuc_entry` to `uint32_t`. We can now parse the int
with `git__strtol64`, which can never return a value bigger than
`UINT32_MAX`, and additionally checking if the returned value is
smaller than zero.

We do not need to handle overflows explicitly here, as
`git__strtol64` returns an error when the returned value would
overflow.

This allows the application to use their own TLS stream, regardless of
the capabilities of libgit2 itself.