[SCM-844] Add support for git submodules to jgit SCM provider

**[Louis Parisot](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=tuxfighter)** opened **[SCM-844](https://issues.apache.org/jira/browse/SCM-844?redirect=false)** and commented

This issue, for JGit provider, is equivalent to SCM-530, which is targeting gitexe provider.
(I'm unable to add the component "maven-scm-provider-jgit" to the issue SCM-530.)

Support for Git submodules is required because performing release:perform on a project that refers to other git repos as submodules cannot succeed because the submodules are not initialized.

---

**Affects:** 1.9.4

**Issue Links:**
- [SCM-530](https://issues.apache.org/jira/browse/SCM-530) Add support for git submodules to git SCM provider

Start by comparing the JGit provider with SCM-530 and identify the provider entry points involved in checkout or release:perform. Done means projects using Git submodules can complete release:perform with their submodules initialized; verify this with the provider's existing tests or a representative release scenario.

Written by the indexing model from the issue text.