fmt-merge-msg: prevent use-after-free with signed tags

fmt-merge-msg: prevent use-after-free with signed tags

When merging a signed tag, fmt_merge_msg_sigs() is responsible for
populating the body of the merge message with the names of the signed
tags, their signatures, and the validity of those signatures.

In 02769437e1 (ssh signing: use sigc struct to pass payload,
2021-12-09), check_signature() was taught to pass the object payload via
the sigc struct instead of passing the payload buffer separately.

In effect, 02769437e1 causes buf, and sigc.payload to point at the same
region in memory. This causes a problem for fmt_tag_signature(), which
wants to read from this location, since it is freed beforehand by
signature_check_clear() (which frees it via sigc's `payload` member).

That makes the subsequent use in fmt_tag_signature() a use-after-free.

As a result, merge messages did not contain the body of any signed tags.
Luckily, they tend not to contain garbage, either, since the result of
strstr()-ing the object buffer in fmt_tag_signature() is guarded:

    const char *tag_body = strstr(buf, "\n\n");
    if (tag_body) {
      tag_body += 2;
      strbuf_add(tagbuf, tag_body, buf + len - tag_body);
    }

Unfortunately, the tests in t6200 did not catch this at the time because
they do not search for the body of signed tags in fmt-merge-msg's
output.

Resolve this by waiting to call signature_check_clear() until after its
contents can be safely discarded. Harden ourselves against any future
regressions in this area by making sure we can find signed tag messages
in the output of fmt-merge-msg, too.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>