preprocess.py: Fixed sequential again, process IDs
preprocess.py: Fixed sequential again, process IDs

- The conditions and control statements in run_sequential break the
  repos download task. Rewrote conditions to separate the two cases.
- Do not use a process ID or tag for the commit_comments non-MPI
  version, since it puts the file in a single subdirectory. Rewrote the
  path appends to actually work with this and not place it at / or try
  to create a directory with an empty name.

timvandermeij/sentiment-analysis,timvandermeij/sentiment-analysis