fastexport: add support for git subrepositories
Questions:
- How to handle case when .gitmodules is already part of the source repository?
- For now, abort. Different options can be implemented as a follow-up.
- A repository with Git subrepositories gets exported differently before and after this change. Should this functionality be opt-in, to ensure stability of the output?
- No, conversion of subrepositories should always be enabled. Because the output changes, the change is marked as breaking.
Edited by Manuel Jacob