blame: simplify prepare_lines()

blame: simplify prepare_lines()

Changing get_next_line() to return the end pointer instead of NULL in
case no newline character is found treats allows us to treat complete
and incomplete lines the same, simplifying the code.  Switching to
counting lines instead of EOLs allows us to start counting at the
first character, instead of having to call get_next_line() first.

Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>