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

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()