Merge branch 'jc/string-list-split' into jch

Merge branch 'jc/string-list-split' into jch

string_list_split*() family of functions have been extended to
simplify common use cases.

* jc/string-list-split:
  string-list: split-then-remove-empty can be done while splitting
  string-list: optionally omit empty string pieces in string_list_split*()
  diff: simplify parsing of diff.colormovedws
  string-list: optionally trim string pieces split by string_list_split*()
  string-list: unify string_list_split* functions
  string-list: align string_list_split() with its _in_place() counterpart
  string-list: report programming error with BUG