Leverage Perceval time window parsing of git logs on parse_gitlog() interface

[carlosparadis]

Perceval git interface provides the following parameters:

  --from-date FROM_DATE
                        fetch items updated since this date (in any ISO 8601
                        format, e.g., 'YYYY-MM-DD HH:mm:SS +|-HH:MM')
  --to-date TO_DATE     fetch items updated before this date (in any ISO 8601
                        format, e.g., 'YYYY-MM-DD HH:mm:SS +|-HH:MM')

parse_gitlog() can have two additional parameters to specify the time range. Unfortunately, the interface does not provide intervals by commit hash, but this may (I hope) save computation time for quick exploration on large projects.