clone: teach --recurse-submodules to optionally take a pathspec

clone: teach --recurse-submodules to optionally take a pathspec

Teach clone --recurse-submodules to optionally take a pathspec argument
which describes which submodules should be recursively initialized and
cloned.  If no pathspec is provided, --recurse-submodules will
recursively initialize and clone all submodules by using a default
pathspec of ".".  In order to construct more complex pathspecs,
--recurse-submodules can be given multiple times.

This also configures the 'submodule.active' configuration option to be
the given pathspec, such that any future invocation of `git submodule
update` will keep up with the pathspec.

Additionally the switch '--recurse' is removed from the Documentation as
well as marked hidden in the options array, to streamline the options
for submodules.  A simple '--recurse' doesn't convey what is being
recursed, e.g. it could mean directories or trees (c.f. ls-tree) In a
lot of other commands we already have '--recurse-submodules' to mean
recursing into submodules, so advertise this spelling here as the
genuine option.

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