rhg: Fall back to Python on unsupported `rhg config <section>`
Printing an entire section (as opposed to a single value with
`rhg config foo.bar`) is not supported yet in Rust only.
Differential Revision: https://phab.mercurial-scm.org/D10098
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 ..