fsck: split ".gitmodules too large" error from parse failure

fsck: split ".gitmodules too large" error from parse failure

Since ed8b10f631 (fsck: check .gitmodules content,
2018-05-02), we'll report a gitmodulesParse error for two
conditions:

  - a .gitmodules entry is not syntactically valid

  - a .gitmodules entry is larger than core.bigFileThreshold

with the intent that we can detect malicious files and
protect downstream clients. E.g., from the issue in
0383bbb901 (submodule-config: verify submodule names as
paths, 2018-04-30).

But these conditions are actually quite different with
respect to that bug:

 - a syntactically invalid file cannot trigger the problem,
   as the victim would barf before hitting the problematic
   code

 - a too-big .gitmodules _can_ trigger the problem. Even
   though it is obviously silly to have a 500MB .gitmodules
   file, the submodule code will happily parse it if you
   have enough memory.

So it may be reasonable to configure their severity
separately. Let's add a new class for the "too large" case
to allow that.

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