Load author file during rcheckin

...because there is fetch of commit even during rcheckin command
Load author file before calling a git-tfs command

- prevent error during reading
- prevent a 2nd read if another file is not specified
- clear collections if should read another file
Reading author file in one place only
Better managing of error when reading author file

Now that the reading is done in only one place, we should :
- stop with an error when an author file is passed and an error occurs
- display a warning if an error occurs when reading the author file in cache
Refactor.
Merge #482