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 This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a