diff: improve lifecycle management of diff queues

The lifecycle management of diff queues is somewhat confusing:

  - For most of the part this can be attributed to `DIFF_QUEUE_CLEAR()`,
    which does not release any memory but rather initializes the queue,
    only. This is in contrast to our common naming schema, where
    "clearing" means that we release underlying memory and then
    re-initialize the data structure such that it is ready to use.

  - A second offender is `diff_free_queue()`, which does not free the
    queue structure itself. It is rather a release-style function.

Refactor the code to make things less confusing. `DIFF_QUEUE_CLEAR()` is
replaced by `DIFF_QUEUE_INIT` and `diff_queue_init()`, while
`diff_free_queue()` is replaced by `diff_queue_release()`. While on it,
adapt callsites where we call `DIFF_QUEUE_CLEAR()` with the intent to
release underlying memory to instead call `diff_queue_clear()` to fix
memory leaks.

This memory leak is exposed by t4211, but plugging it alone does not
make the whole test suite pass.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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