Allow to prepare a git commit message

Add a command to prepare a git commit message from to-be-commited changelog part files content.

The git commit message should adhere to the following git format:

summary
blank line
rest of description

Proposed algorithm:

1. Find Added partlog files
2. If there is only one, use that one
3. Get the main one from the command line or bailout and ask to precise which one is the main one
4. Extract first line of such a file and use it as the main title
5. Parse the title to extract message (ref) if there is a ref
6. Construct the commit message:
   1. a title in the form ref: message.
      Prepend the word Ref before ref if ref starts with a # (because it will be seen as a comment by git).
      Remove any - from the start of message if any.
   2. a blank line
   3. the rest of the main file as-is
   4. the rest of other files as-is (using category and lexicographic order)