Revert "Branching creates a '.branchedFrom' file containing infos about original branch and commit"

This reverts commit 498b0717310687781820991959cb9a33fdac427a.

This feature needs to be more thought about... the .brached_from file
should not be overwritten if you merge another branch into it...
I don't see how that could be easily done, since 'merge=ours' seems
to only be relevant in cases of merge conflicts...