The --recurse-submodules command line parameter has existed for some
time but it has no config file equivalent.

Following the style of the corresponding parameter for git fetch, let's
invent push.recurseSubmodules to provide a default for this
parameter. This also requires the addition of --recurse-submodules=no to
allow the configuration to be overridden on the command line when
required.

The most straightforward way to implement this appears to be to make
push use code in submodule-config in a similar way to fetch.

Signed-off-by: Mike Crowe <mac@mcrowe.com>
Signed-off-by: Jeff King <peff@peff.net>