tests/test-diff-newlines.t

rust: iterator bindings to C code

In this changeset, still made of Rust code only,
we expose the Rust iterator for instantiation and
consumption from C code.

The idea is that both the index and index_get_parents()
will be passed from the C extension, hence avoiding a hard
link dependency to parsers.so, so that the crate can
still be built and tested independently.

On the other hand, parsers.so will use the symbols
defined in this changeset.

  $ hg init

  $ "$PYTHON" -c 'open("a", "wb").write(b"confuse str.splitlines\nembedded\rnewline\n")'
  $ hg ci -Ama -d '1 0'
  adding a

  $ echo clean diff >> a
  $ hg ci -mb -d '2 0'

  $ hg diff -r0 -r1
  diff -r 107ba6f817b5 -r 310ce7989cdc a
  --- a/a	Thu Jan 01 00:00:01 1970 +0000
  +++ b/a	Thu Jan 01 00:00:02 1970 +0000
  @@ -1,2 +1,3 @@
   confuse str.splitlines
   embedded\r (no-eol) (esc)
  newline
  +clean diff