rust: do a clippy pass
This is the result of running `cargo clippy` on hg-core/hg-cpython and fixing
the lints that do not require too much code churn (and would warrant a separate
commit/complete refactor) and only come from our code (a lot of warnings in
hg-cpython come from `rust-cpython`).
Most of those were good lints, two of them was the linter not being smart
enough (or compiler to get up to `clippy`'s level depending on how you see it).
Maybe in the future we could have `clippy` be part of the CI.
Differential Revision: https://phab.mercurial-scm.org/D8635
#require test-repo jshint hg10
$ . "$TESTDIR/helpers-testrepo.sh"
run jshint on all tracked files ending in .js except vendored dependencies
$ cd "`dirname "$TESTDIR"`"
$ testrepohg locate 'set:**.js' \
> 2>/dev/null \
> | xargs jshint