magit-cherry.el: shorten long lines

And refactor `magit-cherry' so that no unneeded local bindings are used.
They are unneeded because they are only used once and other text already
sufficiently documents their purpose.
magit-show-commit: set default-directory to top of repo

Otherwise when `magit-show-commit' is called while the
`default-directory' is not the toplevel of the repository
later visiting items from the commit buffer would fail.
Merge branch 'master' into next
remove cl-lib submodule