submodule-config: verify submodule names as paths

Submodule "names" come from the untrusted .gitmodules file,
but we blindly append them to $GIT_DIR/modules to create our
on-disk repo paths. This means you can do bad things by
putting "../" into the name (among other things).

Let's sanity-check these names to avoid building a path that
can be exploited. There are two main decisions:

  1. What should the allowed syntax be?

     It's tempting to reuse verify_path(), since submodule
     names typically come from in-repo paths. But there are
     two reasons not to:

       a. It's technically more strict than what we need, as
          we really care only about breaking out of the
          $GIT_DIR/modules/ hierarchy.  E.g., having a
          submodule named "foo/.git" isn't actually
          dangerous, and it's possible that somebody has
          manually given such a funny name.

       b. Since we'll eventually use this checking logic in
          fsck to prevent downstream repositories, it should
          be consistent across platforms. Because
          verify_path() relies on is_dir_sep(), it wouldn't
          block "foo\..\bar" on a non-Windows machine.

  2. Where should we enforce it? These days most of the
     .gitmodules reads go through submodule-config.c, so
     I've put it there in the reading step. That should
     cover all of the C code.

     We also construct the name for "git submodule add"
     inside the git-submodule.sh script. This is probably
     not a big deal for security since the name is coming
     from the user anyway, but it would be polite to remind
     them if the name they pick is invalid (and we need to
     expose the name-checker to the shell anyway for our
     test scripts).

     This patch issues a warning when reading .gitmodules
     and just ignores the related config entry completely.
     This will generally end up producing a sensible error,
     as it works the same as a .gitmodules file which is
     missing a submodule entry (so "submodule update" will
     barf, but "git clone --recurse-submodules" will print
     an error but not abort the clone.

     There is one minor oddity, which is that we print the
     warning once per malformed config key (since that's how
     the config subsystem gives us the entries). So in the
     new test, for example, the user would see three
     warnings. That's OK, since the intent is that this case
     should never come up outside of malicious repositories
     (and then it might even benefit the user to see the
     message multiple times).

Credit for finding this vulnerability and the proof of
concept from which the test script was adapted goes to
Etienne Stalmans.

Signed-off-by: Jeff King <peff@peff.net>

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