hggit: fix backwards compatibility with extrainmessage
When hg-git-rename-source was introduced, it was added unconditionally in order to not break bidirectionality, however, that only applies if debugextrainmessage is not set, if it's set 'HG:rename-source' is not added.
The hg-git-rename-source change broke compatibility with git-remote-hg, the following patch restores it back.
Fixes #281 (closed).