Fix os.rename in git source element to correctly handle error codes

According to the documentation
(https://www.unix.com/man-page/POSIX/3posix/rename/), when the directory
already is there, either EEXIST or ENOTEMPTY could be thrown.

Previously only ENOTEMPTY was checked.

Done:
  - Separated the move into its own function
  - Check for both errors
  - Create unit tests for it, covering most test cases