tests/test-empty-dir.t

tests: restore 'python' and '$TESTDIR/' for dummyssh invocation

This is a backout of 46727fea7a00, and a partial backout of c3ecbf694904.

Windows won't execute 'dummyssh' directly, presumably because CreateProcess()
doesn't know how to execute a bash script:

   $ hg clone -e "dummyssh" ssh://user@dummy/cloned sshclone
   remote: 'dummyssh' is not recognized as an internal or external command,
   remote: operable program or batch file.
   abort: no suitable response from remote hg!
   [255]

With the restoration of python as the executable, $TESTDIR needs to be restored
for these invocations, because python won't search $PATH for 'dummyssh':

   $ hg clone -e "python dummyssh" ssh://user@dummy/cloned sshclone
   remote: python: can't open file 'dummyssh': [Errno 2] No such file or directory
   abort: no suitable response from remote hg!
   [255]

  $ 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