tests/test-backwards-remove.t

testlib: move the prune alias into a shell script for Windows

Cramming all of this directly into an alias doesn't play nicely on Windows.
Various test-exchange-obsmarkers-case-XX.t were failing with:

    abort: cannot select revision when creating marker

It turned out that inside debugcommands.debugobsolete(), the following differed
from Linux (where they were empty, at least in the case I debugged):

    'rev': ['.`;'],
    'template': "'{node}\\n'"

  $ hg init
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ ls
  a
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ hg co 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

B should disappear

  $ ls
  a