range-diff: split lines manually

range-diff: split lines manually

Currently range-diff uses the 'strbuf_getline()' function for doing
its line by line processing.  In a future patch we want to do parts of
that parsing using the 'parse_git_diff_header()' function.  That
function does its own line by line reading of the input, and doesn't
use strbufs.  This doesn't match with how we do the line-by-line
processing in range-diff currently.

Switch range-diff to do our own line by line parsing, so we can re-use
the 'parse_git_diff_header()' function later.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>