completion: complete 'submodule.*' config variables

completion: complete 'submodule.*' config variables

In the Bash completion script, function
__git_complete_config_variable_name completes config variables and has
special logic to deal with config variables involving user-defined
names, like branch.<name>.* and remote.<name>.*.

This special logic is missing for submodule-related config variables.
Add the appropriate branches to the case statement, making use of the
in-tree '.gitmodules' to list relevant submodules.

Add corresponding tests in t9902-completion.sh, making sure we complete
both first level submodule config variables as well as second level
variables involving submodule names.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>