tests/test-cat.t

push: move obsolescence marker exchange in the exchange module

The obsolescence marker exchange code was already extracted during a previous
cycle. We are moving the extracted functio in this module. This function will
read and write data in the `pushoperation` object and I prefer to have all core
function collaborating through this object in the same place.

This changeset is pure code movement only. Code change for direct consumption of
the `pushoperation` object will come later.

  $ hg init
  $ echo 0 > a
  $ echo 0 > b
  $ hg ci -A -m m
  adding a
  adding b
  $ hg rm a
  $ hg cat a
  0
  $ hg cat --decode a # more tests in test-encode
  0
  $ echo 1 > b
  $ hg ci -m m
  $ echo 2 > b
  $ hg cat -r 0 a
  0
  $ hg cat -r 0 b
  0
  $ hg cat -r 1 a
  a: no such file in rev 7040230c159c
  [1]
  $ hg cat -r 1 b
  1

Test fileset

  $ echo 3 > c
  $ hg ci -Am addmore c
  $ hg cat 'set:not(b) or a'
  3
  $ hg cat 'set:c or b'
  1
  3