Allow empty summary commit at the end of rmerge()

This should allow an empty merge summary commit to be created if:
- there is no submodule in the repository
- there are submodules but none of them is updated during the merge operation
Pass allow_empty to remerge() and set it to False for UpdateSubmodules
Merge pull request #149 from sbesson/146_commit_message

Allow empty summary commit at the end of rmerge()