worktree: check if a submodule uses worktrees

In a later patch we want to move around the the git directory of
a submodule. Both submodules as well as worktrees are involved in
placing git directories at unusual places, so their functionality
may collide. To react appropriately to situations where worktrees
in submodules are in use, offer a new function to query the
a submodule if it uses the worktree feature.

An earlier approach:
  "Implement submodule_get_worktrees and just count them", however:
  This can be done cheaply (both in new code to write as well as run time)
  by obtaining the list of worktrees based off that submodules git
  directory. However as we have loaded the variables for the current
  repository, the values in the submodule worktree
  can be wrong, e.g.
  * core.ignorecase may differ between these two repositories
  * the ref resolution is broken (refs/heads/branch in the submodule
    resolves to the sha1 value of the `branch` in the current repository
    that may not exist or have another sha1)

The implementation here is just checking for any files in
$GIT_COMMON_DIR/worktrees for the submodule, which ought to be sufficient
if the submodule is using the current repository format, which we also
check.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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