Various minor bug fixes and improvements.

* Bumping a version number resets the lesser version parts to zero.
* If an invalid index is entered in the prompt, TINA no longer crashes.
* Marking a changed repo as unchanged resets the version to the
  original.
* A brief summary of the change made is printed out during the
  interactive prompt.
Added better support for command-line arguments.

Support for both short and long arguments has been included (running
tina -h or tina --help will elaborate).  Interactive mode is now
initiated via a command-line flag, and the .tina/ directory is only
retained if the corresponding command-line flag is set.
Removed comparator that isn't used anymore.