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
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[10]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate