add start_commit and end_commit parameter

[JonathanGuerne]

• add start_commit and end_commit parameters
  • both optional
  • override before and after when used
• retrieve a commit date with new function get_commit_timestamp
• ensure the commit hash has the correct length (or output an error)
• transform hash to their shorter form
• extract time from commit information
• ensure start_commit and end_commit are in correct order
• if start_commit and end_commit are identical retrieve only this commits data