tests/test-backwards-remove

document and fix findincoming

- add documentation about what the function does, notably
  the fact that it updates 'base'
- transform the workflow to a more simple 'if elif elif else'
- do not call remote.branches if not necessary
- some nodes where missing in 'base' (from what I understand,
  if the root of a branch is missing but one parent is present,
  the parent should be in 'base')
- add a testcase for an incorrect outgoing that is fixed by
  this cset
- add a testcase for an empty group bug, it needs fixing

#!/bin/sh
hg init
echo This is file a1 > a
hg add a
hg commit -m "commit #0" -d "1000000 0"
ls
echo This is file b1 > b
hg add b
hg commit -m "commit #1" -d "1000000 0"
hg co 0
# B should disappear
ls