persistent-nodemap: enable the feature by default when using Rust
As discussed at the 5.6 sprint, we can make it enabled by default, but only for
Rust installation.
Differential Revision: https://phab.mercurial-scm.org/D9765
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 ..