- addfile():
* contains the complete logic for adding a file (=> simplified addFiles(...))
* semantic fixes
- delete_file():
* semantic fixes
- different handling of newly added/replaced files:
* added/replaced files are tracked in the .osc/_to_be_added file
- introduced new file state 'R': 'R' == "replaced"
* usecase: osc rm <file>; osc add <file> => new state is 'R'
* conceptually 'R' is equal to 'A'
- adapted revert() to support the new state (+ some other minor fixes)
- added testcases for addfile() and delete_file()