Submodules: Add the new "fetch" config option for fetch and pull

Submodules: Add the new "fetch" config option for fetch and pull

The new boolean "fetch" config option controls the default behavior for
"git fetch" and "git pull". It specifies if these commands should recurse
into submodules and fetch new commits there too and can be set separately
for each submodule.

The .gitmodules file is parsed for "submodule.<name>.fetch" entries before
looking for them in .git/config. Thus settings found in .git/config will
override those from .gitmodules, thereby allowing the local developer to
ignore settings given by the remote side while also letting upstream set
defaults for those users who don't have special needs.

This configuration can be overridden by the command line option
"--[no-]recursive" of "git fetch" and "git pull".

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>