As with urls, submodule paths with dashes are ignored by
git, but may end up confusing older versions. Detecting them
via fsck lets us prevent modern versions of git from being a
vector to spread broken .gitmodules to older versions.

Compared to blocking leading-dash urls, though, this
detection may be less of a good idea:

  1. While such paths provide confusing and broken results,
     they don't seem to actually work as option injections
     against anything except "cd". In particular, the
     submodule code seems to canonicalize to an absolute
     path before running "git clone" (so it passes
     /your/clone/-sub).

  2. It's more likely that we may one day make such names
     actually work correctly. Even after we revert this fsck
     check, it will continue to be a hassle until hosting
     servers are all updated.

On the other hand, it's not entirely clear that the behavior
in older versions is safe. And if we do want to eventually
allow this, we may end up doing so with a special syntax
anyway (e.g., writing "./-sub" in the .gitmodules file, and
teaching the submodule code to canonicalize it when
comparing).

So on balance, this is probably a good protection.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

We recently banned submodule urls that look like
command-line options. This is the matching change to ban
leading-dash paths.

As with the urls, this should not break any use cases that
currently work. Even with our "--" separator passed to
git-clone, git-submodule.sh gets confused. Without the code
portion of this patch, the clone of "-sub" added in t7417
would yield results like:

    /path/to/git-submodule: 410: cd: Illegal option -s
    /path/to/git-submodule: 417: cd: Illegal option -s
    /path/to/git-submodule: 410: cd: Illegal option -s
    /path/to/git-submodule: 417: cd: Illegal option -s
    Fetched in submodule path '-sub', but it did not contain b56243f8f4eb91b2f1f8109452e659f14dd3fbe4. Direct fetching of that commit failed.

Moreover, naively adding such a submodule doesn't work:

  $ git submodule add $url -sub
  The following path is ignored by one of your .gitignore files:
  -sub

even though there is no such ignore pattern (the test script
hacks around this with a well-placed "git mv").

Unlike leading-dash urls, though, it's possible that such a
path _could_ be useful if we eventually made it work. So
this commit should be seen not as recommending a particular
policy, but rather temporarily closing off a broken and
possibly dangerous code-path. We may revisit this decision
later.

There are two minor differences to the tests in t7416 (that
covered urls):

  1. We don't have a "./-sub" escape hatch to make this
     work, since the submodule code expects to be able to
     match canonical index names to the path field (so you
     are free to add submodule config with that path, but we
     would never actually use it, since an index entry would
     never start with "./").

  2. After this patch, cloning actually succeeds. Since we
     ignore the submodule.*.path value, we fail to find a
     config stanza for our submodule at all, and simply
     treat it as inactive. We still check for the "ignoring"
     message.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>