commit, tag: don't set parsed bit for parse failures

commit, tag: don't set parsed bit for parse failures

If we can't parse a commit, then parse_commit() will return an error
code. But it _also_ sets the "parsed" flag, which tells us not to bother
trying to re-parse the object. That means that subsequent parses have no
idea that the information in the struct may be bogus.  I.e., doing this:

  parse_commit(commit);
  ...
  if (parse_commit(commit) < 0)
          die("commit is broken");

will never trigger the die(). The second parse_commit() will see the
"parsed" flag and quietly return success.

There are two obvious ways to fix this:

  1. Stop setting "parsed" until we've successfully parsed.

  2. Keep a second "corrupt" flag to indicate that we saw an error (and
     when the parsed flag is set, return 0/-1 depending on the corrupt
     flag).

This patch does option 1. The obvious downside versus option 2 is that
we might continually re-parse a broken object. But in practice,
corruption like this is rare, and we typically die() or return an error
in the caller. So it's OK not to worry about optimizing for corruption.
And it's much simpler: we don't need to use an extra bit in the object
struct, and callers which check the "parsed" flag don't need to learn
about the corrupt bit, too.

There's no new test here, because this case is already covered in t5318.
Note that we do need to update the expected message there, because we
now detect the problem in the return from "parse_commit()", and not with
a separate check for a NULL tree. In fact, we can now ditch that
explicit tree check entirely, as we're covered robustly by this change
(and the previous recent change to treat a NULL tree as a parse error).

We'll also give tags the same treatment. I don't know offhand of any
cases where the problem can be triggered (it implies somebody ignoring a
parse error earlier in the process), but consistently returning an error
should cause the least surprise.

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