repoview: move subsettable in a dedicated module
The dictionary got moved in `branchmap` to avoid import cycle. However, we are
about to needs it in repoview too. So we introduce a now module to define that
that mapping.
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..