Strange sort-of-regression in a single lint: clippy::redundant-clone

[mulkieran]

The evidence is here:

https://travis-ci.org/stratis-storage/stratisd/builds/633945478?utm_source=github_status&utm_medium=notification

There is one failure, clippy task on 1.40. There is just a single error reported. What makes this interesting is:

1. We ran the identical clippy test on Rust beta and it succeeded. That's the bottom mandatory Travis target.
2. we know it wasn't a failure on clippy task for 1.39 which was run on the same code just shortly before:

https://travis-ci.org/stratis-storage/stratisd/builds/633803737utm_medium=notification&utm_source=github_status.

So this correct clippy error appeared in 1.40, and went away again in 1.41.

[mulkieran]

I made up a new PR: stratis-storage/stratisd#1773 that has exactly the code that causes the problem.