tests/test-mq-qsave.t

rust: simply use TryInto to convert slice to array

Since our rust module depends on TryInto, there's no point to avoid using it.

While rewriting copy_into_array(), I noticed CPython interface doesn't check
the length of the p1/p2 values, which is marked as TODO.

  $ echo "[extensions]" >> $HGRCPATH
  $ echo "mq=" >> $HGRCPATH

  $ hg init

  $ echo 'base' > base
  $ hg ci -Ambase
  adding base

  $ hg qnew -mmqbase mqbase

  $ hg qsave
  $ hg qrestore 2
  restoring status: hg patches saved state