[BUG?] gitlink without .gitmodules no longer fails recursive clone

[Jeff King <peff@peff.net>]

While running some regression tests with v2.13, I noticed an odd
behavior. If I create a repository where there's a gitlink with no
matching .gitmodules entry:

  git init repo
  cd repo
  n10=1234abcdef
  n40=$n10$n10$n10$n10
  git update-index --add --cacheinfo 160000 $n40 foo
  git commit -m "gitlink without .gitmodule entry"

and then I clone it recursively with v2.12, it fails:

  $ git.v2.12.3 clone --recurse-submodules . dst; echo exit=$?
  Cloning into 'dst'...
  done.
  fatal: No url found for submodule path 'foo' in .gitmodules
  exit=128

But with v2.13, it silently ignores the submodule:

  $ git.v2.13.1 clone --recurse-submodules . dst; echo exit=$?
  Cloning into 'dst'...
  done.
  exit=0

This bisects to your bb62e0a99 (clone: teach --recurse-submodules to
optionally take a pathspec, 2017-03-17). That patch just sets
submodule.active by default, so I think the real issue is probably in
a086f921a (submodule: decouple url and submodule interest, 2017-03-17).

I also wasn't sure if this might be intentional. I.e., that we'd just
consider gitlink entries which aren't even configured as not-submodules
and ignore them. I couldn't certainly see an argument for moving in that
direction, but it is different than what we used to do. But I couldn't
find anything in any of the commit messages that mentioned this either
way, so I figured I'd punt and ask. :)

-Peff