collie init should use an existing git repo if CWD is a git repo already

[JohannesRudolph]

A lot of new collie users create a repo e.g. on GitHub first, check it out locally and then run collie init.

In this case, collie does currently generate a nested git repo. This is confusing for first time users.

collie init should use an existing git repo if CWD is a git repo already

[JohannesRudolph]

todo here: it should add the collie specific entries to an existing .gitignore file as well