Merge branch 'js/libify-require-clean-work-tree'

The require_clean_work_tree() helper was recreated in C when "git pull" was rewritten from shell; the helper is now made available to other callers in preparation for upcoming "rebase -i" work. * js/libify-require-clean-work-tree: wt-status: begin error messages with lower-case wt-status: teach has_{unstaged,uncommitted}_changes() about submodules wt-status: export also the has_un{staged,committed}_changes() functions wt-status: make the require_clean_work_tree() function reusable pull: make code more similar to the shell script again pull: drop confusing prefix parameter of die_on_unclean_work_tree()