tests/test-absorb-unfinished.t

graftcopies: use _filter() for filtering out invalid copies

`graftcopies()` (formerly called `duplicatecopies()`) checked that the
copy destination existed in the working copy, but it didn't check that
copy source existed in the parent of the working copy. In
`test-graft.t` we can see that as warnings about not finding ancestors
of the copied files, and also empty commits getting created.

This patch uses the existing `_filter()` function for filtering out
invalid copies. In addition to the aforementioned types, that also
includes copies where source and destination is the same.

Differential Revision: https://phab.mercurial-scm.org/D7859

  $ cat >> $HGRCPATH << EOF
  > [extensions]
  > absorb=
  > EOF

Abort absorb if there is an unfinished operation.

  $ hg init abortunresolved
  $ cd abortunresolved

  $ echo "foo1" > foo.whole
  $ hg commit -Aqm "foo 1"

  $ hg update null
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ echo "foo2" > foo.whole
  $ hg commit -Aqm "foo 2"

  $ hg --config extensions.rebase= rebase -r 1 -d 0
  rebasing 1:c3b6dc0e177a "foo 2" (tip)
  merging foo.whole
  warning: conflicts while merging foo.whole! (edit, then use 'hg resolve --mark')
  unresolved conflicts (see hg resolve, then hg rebase --continue)
  [1]

  $ hg --config extensions.rebase= absorb
  abort: rebase in progress
  (use 'hg rebase --continue' or 'hg rebase --abort')
  [255]