sparse-checkout: always free "line" strbuf after reading input

sparse-checkout: always free "line" strbuf after reading input

In add_patterns_from_input(), we may read lines from a file with a loop
like this:

  while (!strbuf_getline(&line, file)) {
	...
	strbuf_to_cone_pattern(&line, pl);
  }
  /* we don't strbuf_release(&line) here! */

This generally is OK because strbuf_to_cone_pattern() consumes the
buffer via strbuf_detach(). But we can leak in a few cases:

  1. We don't always consume the buffer! If the line ends up empty after
     trimming, we leave strbuf_to_cone_pattern() without detaching. In
     most cases this is OK, because a subsequent getline() call will use
     the same buffer. But if you had an empty line at the end of file,
     for example, it would leak.

  2. Even if strbuf_to_cone_pattern() always consumed the buffer,
     there's a subtle issue with strbuf_getline(). As we saw in
     94e2aa555e (strbuf: fix leak when `appendwholeline()` fails with
     EOF, 2024-05-27), it's possible for it to return EOF with an
     allocated buffer (e.g., if the underlying getdelim() call saw an
     error). So we should always strbuf_release() after finishing a read
     loop like this.

Note that even the code to read patterns from argv has the same problem.
Because that also uses strbuf_to_cone_pattern(), we stuff each argv
entry into a strbuf. It uses the same "line" strbuf as the getline code,
but we should position the strbuf_release() to cover both code paths.

This fixes at least 9 leaks found in t1091.

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