[SCM-530] Add support for git submodules to git SCM provider

[Tomasz Krakowiak <ni...@gmail.com>]

Hello,
I would like to contribute to maven. I’m maintain a few projects using maven and git with submodules. I have each artifact in separated repository and some aggregating poms in repositories with submodules. Deploying snapshots works like a charm, but when it comes to release it’s really time-consuming releasing each module separately. Common workaround for this issue involves enabling commitByProject and disabling pushChanges option. Still, it is workaround.

I already described how I think it should work here - https://jira.codehaus.org/browse/SCM-530 . 

One thing troubles me:
- I would like to extend AbstractGitScmProvider in maven-scm-provider-git-commons so it would have abstract List<GitSubmodule> listSubmodules(boolean) method.
- Create abstract class for git checkout command and pass AbstractGitScmProvider object to it in constructor. (I might also extract interface from AbstractGitScmProvider with listSubmodules method).

When I dug through the SCM code I haven’t found suchlike practices. Everything is very schematic. Therefore maybe I should implement it abstract git checkout command with abstract List<GitSubmodule> listSubmodules(boolean) method?

I would be glad for reply, because I’m new here and I don’t know how things are getting done around here nor who supervise this module.

Regards,
Tomasz