submodule: decouple url and submodule interest

Currently the submodule.<name>.url config option is used to determine if
a given submodule is of interest to the user.  This ends up being
cumbersome in a world where we want to have different submodules checked
out in different worktrees or a more generalized mechanism to select
which submodules are of interest.

In a future with worktree support for submodules, there will be multiple
working trees, each of which may only need a subset of the submodules
checked out.  The URL (which is where the submodule repository can be
obtained) should not differ between different working trees.

It may also be convenient for users to more easily specify groups of
submodules they are interested in as opposed to running "git submodule
init <path>" on each submodule they want checked out in their working
tree.

To this end two config options are introduced, submodule.active and
submodule.<name>.active.  The submodule.active config holds a pathspec
that specifies which submodules should exist in the working tree.  The
submodule.<name>.active config is a boolean flag used to indicate if
that particular submodule should exist in the working tree.

Its important to note that submodule.active functions differently than
the other configuration options since it takes a pathspec.  This allows
users to adopt at least two new workflows:

  1. Submodules can be grouped with a leading directory, such that a
     pathspec e.g. 'lib/' would cover all library-ish modules to allow
     those who are interested in library-ish modules to set
     "submodule.active = lib/" just once to say any and all modules in
     'lib/' are interesting.

  2. Once the pathspec-attribute feature is invented, users can label
     submodules with attributes to group them, so that a broad pathspec
     with attribute requirements, e.g. ':(attr:lib)', can be used to say
     any and all modules with the 'lib' attribute are interesting.
     Since the .gitattributes file, just like the .gitmodules file, is
     tracked by the superproject, when a submodule moves in the
     superproject tree, the project can adjust which path gets the
     attribute in .gitattributes, just like it can adjust which path has
     the submodule in .gitmodules.

Neither of these two additional configuration options solve the problem
of wanting different submodules checked out in different worktrees
because multiple worktrees share .git/config.  Only once per-worktree
configurations become a reality can this be solved, but this is a
necessary preparatory step for that future.

Given these multiple ways to check if a submodule is of interest, the
more fine-grained submodule.<name>.active option has the highest order
of precedence followed by the pathspec check against submodule.active.
To ensure backwards compatibility, if neither of these options are set,
git falls back to checking the submodule.<name>.url option to determine
if a submodule is interesting.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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