mktree: there are only two possible line terminations

mktree: there are only two possible line terminations

The program by default reads LF terminated lines, with an option to
use NUL terminated records.  Instead of pretending that there can be
other useful values for line_termination, use a boolean variable,
nul_term_line, to tell if NUL terminated records are used, and
switch between strbuf_getline_{lf,nul} based on it.

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