pull: move `fetch` subset into the object
Tree discovey use a `fetch` variable to know what is being pulled. We move this
information in the `pulloperation` object. This make it possible to extract the
changeset pulling logic into its own function.
$ 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