The preliminary clean-up for jc/peace-with-crlf topic.

* jc/strbuf-getline:
  strbuf: give strbuf_getline() to the "most text friendly" variant
  checkout-index: there are only two possible line terminations
  update-index: there are only two possible line terminations
  check-ignore: there are only two possible line terminations
  check-attr: there are only two possible line terminations
  mktree: there are only two possible line terminations
  strbuf: introduce strbuf_getline_{lf,nul}()
  strbuf: make strbuf_getline_crlf() global
  strbuf: miniscule style fix