Add the option "--ignore-submodules" to "git status"

Add the option "--ignore-submodules" to "git status"

In some use cases it is not desirable that "git status" considers
submodules that only contain untracked content as dirty. This may happen
e.g. when the submodule is not under the developers control and not all
build generated files have been added to .gitignore by the upstream
developers. Using the "untracked" parameter for the "--ignore-submodules"
option disables checking for untracked content and lets git diff report
them as changed only when they have new commits or modified content.

Sometimes it is not wanted to have submodules show up as changed when they
just contain changes to their work tree (this was the behavior before
1.7.0). An example for that are scripts which just want to check for
submodule commits while ignoring any changes to the work tree. Also users
having large submodules known not to change might want to use this option,
as the - sometimes substantial - time it takes to scan the submodule work
tree(s) is saved when using the "dirty" parameter.

And if you want to ignore any changes to submodules, you can now do that
by using this option without parameters or with "all" (when the config
option status.submodulesummary is set, using "all" will also suppress the
output of the submodule summary).

A new function handle_ignore_submodules_arg() is introduced to parse this
option new to "git status" in a single location, as "git diff" already
knew it.

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