rust: run `cargo clippy`
These automatic fixes are good to have because they make the code
more idiomatic and less surprising.
The transform from `sort` -> `sort_unstable` is questionable, but this is
only in a test, so it doesn't matter in our case.
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2