makefile: run Rust tests if cargo is installed While no particular minimum toolchain version is targeted as of yet, this serves as a first step to make more people/machines run the Rust tests.

merge: hint about using `hg resolve` for resolving conflicts This was suggested by one of our users at Google. Makes sense to me. Differential Revision: https://phab.mercurial-scm.org/D6755

rust-dirstate: remove test case for DirsMultiset::new(Manifest, Some) It's no longer possible.

rust-dirstate: split DirsMultiset constructor per input type Since skip_state only applies to dirstate, it doesn't make sense to unify these constructors and dispatch by enum.

rust-dirstate: remove excessive clone() of parameter and return value I think pass-by-ref is preferred in general.

rust-dirstate: handle invalid length of p1/p2 parameters It uses match syntax since map_err() failed to deduce the argument type.

rust: simply use TryInto to convert slice to array Since our rust module depends on TryInto, there's no point to avoid using it. While rewriting copy_into_array(), I noticed CPython interface doesn't check the length of the p1/p2 values, which is marked as TODO.

rust-dirstate: use PARENT_SIZE constant where appropriate

rust-dirstate: rename NULL_REVISION to NULL_ID which isn't a revision number

rust-dirstate: remove repetition in array literal