tests/test-changelog-exec.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'.

b51a8138292a introduced a regression where we would mention in the
changelog executable files added by the second parent of a merge. Test
that that doesn't happen anymore

  $ "$TESTDIR/hghave" execbit || exit 80

  $ hg init repo
  $ cd repo
  $ echo foo > foo
  $ hg ci -qAm 'add foo'

  $ echo bar > bar
  $ chmod +x bar
  $ hg ci -qAm 'add bar'

manifest of p2:

  $ hg manifest
  bar
  foo

  $ hg up -qC 0
  $ echo >> foo
  $ hg ci -m 'change foo'
  created new head

manifest of p1:

  $ hg manifest
  foo

  $ hg merge
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg ci -m 'merge'

this should not mention bar:

  $ hg tip -v
  changeset:   3:ef2fc9b4a51b
  tag:         tip
  parent:      2:ed1b79f46b9a
  parent:      1:d394a8db219b
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  description:
  merge
  
  

  $ hg debugindex bar
     rev    offset  length  ..... linkrev nodeid       p1           p2 (re)
       0         0       5  .....       1 b004912a8510 000000000000 000000000000 (re)

  $ cd ..