tests/test-empty-dir.t

debugindex etc.: add --changelog and --manifest options

These open the changelog and manifest, respectively, directly so you don't
need to specify the path.

The options have been added to debugindex, debugdata and debugrevlog.

The patch also fixes some minor usage-related bugs.

  $ hg init

  $ echo 123 > a
  $ hg add a
  $ hg commit -m "first" a

  $ mkdir sub
  $ echo 321 > sub/b
  $ hg add sub/b
  $ hg commit -m "second" sub/b

  $ cat sub/b
  321

  $ hg co 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

  $ cat sub/b 2>/dev/null || echo "sub/b not present"
  sub/b not present

  $ test -d sub || echo "sub not present"
  sub not present