tests/test-import-unknown.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'.

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

  $ hg init test
  $ cd test
  $ echo a > changed
  $ echo a > removed
  $ echo a > source
  $ hg ci -Am addfiles
  adding changed
  adding removed
  adding source
  $ echo a >> changed
  $ echo a > added
  $ hg add added
  $ hg rm removed
  $ hg cp source copied
  $ hg diff --git > ../unknown.diff

Test adding on top of an unknown file

  $ hg up -qC 0
  $ hg purge
  $ echo a > added
  $ hg import --no-commit ../unknown.diff
  applying ../unknown.diff
  file added already exists
  1 out of 1 hunks FAILED -- saving rejects to file added.rej
  abort: patch failed to apply
  [255]

Test modifying an unknown file

  $ hg revert -aq
  $ hg purge
  $ hg rm changed
  $ hg ci -m removechanged
  $ echo a > changed
  $ hg import --no-commit ../unknown.diff
  applying ../unknown.diff
  abort: cannot patch changed: file is not tracked
  [255]

Test removing an unknown file

  $ hg up -qC 0
  $ hg purge
  $ hg rm removed
  $ hg ci -m removeremoved
  created new head
  $ echo a > removed
  $ hg import --no-commit ../unknown.diff
  applying ../unknown.diff
  abort: cannot patch removed: file is not tracked
  [255]

Test copying onto an unknown file

  $ hg up -qC 0
  $ hg purge
  $ echo a > copied
  $ hg import --no-commit ../unknown.diff
  applying ../unknown.diff
  abort: cannot create copied: destination already exists
  [255]

  $ cd ..