sparse-checkout: reuse --stdin buffer when reading patterns

sparse-checkout: reuse --stdin buffer when reading patterns

When we read patterns from --stdin, we loop on strbuf_getline(), and
detach each line we read to pass into add_pattern(). This used to be
necessary because add_pattern() required that the pattern strings remain
valid while the pattern_list was in use. But it also created a leak,
since we didn't record the detached buffers anywhere else.

Now that add_pattern() has been modified to make its own copy of the
strings, we can stop detaching and fix the leak. This fixes 4 leaks
detected in t1091.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>