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