rust: fix warning about unnecessary mut
If there's a reason to use mut (like compability with older
compilers), then we should stick `#[allow(unused_mut)]` on the
declaration.
Differential Revision: https://phab.mercurial-scm.org/D8538
upgrade: support upgrade and downgrade from persistent nodemap
The requirements is now recognised and dealt with and the associated files
properly handled.
The persistent nodemap should be ready for usage in the field now.
Differential Revision: https://phab.mercurial-scm.org/D8431