tests/test-copy-move-merge.t

basectx: add an empty class that will be used as a parent of all contexts

At the moment, there is no simple way to check if an object is a context
because there is no common parent class. If there were, we could use
'isinstance' everywhere. Simply having memctx inherit from workingctx or
changectx would allow the use of 'isinstance' but that could lead to some
confusing situations of reading the code since we have three distinct concepts
of a context:

- changectx represents a changeset *already* in the repo, and is therefore immutable
- workingctx represents changes on disk in the working directory
- memctx represents changes solely in memory which may or may not be on disk

Therefore, I propose refactoring context.py to have all three contexts inherit
from a parent class 'basectx'.

  $ hg init t
  $ cd t

  $ echo 1 > a
  $ hg ci -qAm "first"

  $ hg cp a b
  $ hg mv a c
  $ echo 2 >> b
  $ echo 2 >> c

  $ hg ci -qAm "second"

  $ hg co -C 0
  1 files updated, 0 files merged, 2 files removed, 0 files unresolved

  $ echo 0 > a
  $ echo 1 >> a

  $ hg ci -qAm "other"

  $ hg merge --debug
    searching for copies back to rev 1
    unmatched files in other:
     b
     c
    all copies found (* = to merge, ! = divergent, % = renamed and deleted):
     src: 'a' -> dst: 'b' *
     src: 'a' -> dst: 'c' *
    checking for directory renames
  resolving manifests
   branchmerge: True, force: False, partial: False
   ancestor: b8bf91eeebbc, local: add3f11052fa+, remote: 17c05bb7fcb6
   a: remote moved to b -> m
    preserving a for resolve of b
   a: remote moved to c -> m
    preserving a for resolve of c
  removing a
  updating: a 1/2 files (50.00%)
  picked tool 'internal:merge' for b (binary False symlink False)
  merging a and b to b
  my b@add3f11052fa+ other b@17c05bb7fcb6 ancestor a@b8bf91eeebbc
   premerge successful
  updating: a 2/2 files (100.00%)
  picked tool 'internal:merge' for c (binary False symlink False)
  merging a and c to c
  my c@add3f11052fa+ other c@17c05bb7fcb6 ancestor a@b8bf91eeebbc
   premerge successful
  0 files updated, 2 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)

file b
  $ cat b
  0
  1
  2

file c
  $ cat c
  0
  1
  2

  $ cd ..