tests/test-double-merge.t

abort: added logic for of hg abort

This is part of `GSoC19` project `Implement abort and
continue commands`. This patch is part of the `abort plan`.

This adds the basic logic for `hg abort`. This command
aborts an multistep operation like graft, histedit, rebase,
merge and unshelve if they are in an unfinished state.

The first part of the logic is determining the unfinished
operation from the state detection API under `statemod`.
This API is extended to support `hg abort` by adding a method
to register the abort logic as a function (here `abortfunc`).

Once the unfinished operation is determined the registered
logic is used to abort the command. The benefit of this kind
of framework is that any new extension developed can support
`hg abort` by registering the command and logic under
statedetection API.

`hg abort` currently supports `--dry-run/-n` flag only.
It is used to dry run `hg abort`

Further patches sequentially add support for `graft`, `rebase`,
`unshelve`, `histedit` and `merge`.

Differential Revision: https://phab.mercurial-scm.org/D6566

  $ hg init repo
  $ cd repo

  $ echo line 1 > foo
  $ hg ci -qAm 'add foo'

copy foo to bar and change both files
  $ hg cp foo bar
  $ echo line 2-1 >> foo
  $ echo line 2-2 >> bar
  $ hg ci -m 'cp foo bar; change both'

in another branch, change foo in a way that doesn't conflict with
the other changes
  $ hg up -qC 0
  $ echo line 0 > foo
  $ hg cat foo >> foo
  $ hg ci -m 'change foo'
  created new head

we get conflicts that shouldn't be there
  $ hg merge -P
  changeset:   1:484bf6903104
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     cp foo bar; change both
  
  $ hg merge --debug
    unmatched files in other:
     bar
    all copies found (* = to merge, ! = divergent, % = renamed and deleted):
     src: 'foo' -> dst: 'bar' *
    checking for directory renames
  resolving manifests
   branchmerge: True, force: False, partial: False
   ancestor: e6dc8efe11cc, local: 6a0df1dad128+, remote: 484bf6903104
   preserving foo for resolve of bar
   preserving foo for resolve of foo
  starting 4 threads for background file closing (?)
   bar: remote copied from foo -> m (premerge)
  picked tool ':merge' for bar (binary False symlink False changedelete False)
  merging foo and bar to bar
  my bar@6a0df1dad128+ other bar@484bf6903104 ancestor foo@e6dc8efe11cc
   premerge successful
   foo: versions differ -> m (premerge)
  picked tool ':merge' for foo (binary False symlink False changedelete False)
  merging foo
  my foo@6a0df1dad128+ other foo@484bf6903104 ancestor foo@e6dc8efe11cc
   premerge successful
  0 files updated, 2 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)

contents of foo
  $ cat foo
  line 0
  line 1
  line 2-1

contents of bar
  $ cat bar
  line 0
  line 1
  line 2-2

  $ cd ..