add option `--fixed-tag` and `--fixed-version`

[copernico]

• add option --fixed-tag and --fixed-version so that the left-hand version/tag of an interval is inferred by the tool and used automatically (with a visible message indicating which tag-version is considered). If it cannot be determined uniquely, the tool will print a message showing all the alternatives and stop. The user can then retry with one of those alternatives, if they wish.

[copernico]

This is a bit trickier than I expected because the function to obtain the "previous tag" is commented out in the current code and would require some rework to be re-integrated. In particular, it currently expects a "git" (repository) object as a parameter, which is then used to get the list of existing tags in the repository and their timestamp. Instead, we'd better obtain this list first and then pass it to the function.

[copernico]

One way to approach this is to imply an interactive mode, whereby the tool would try to identify the previous tag/version but the user would have to confirm to proceed.

[copernico]

It seems that some suitable git tricks might help us here.
For instance, this command returns all the commits annotated by some tag, and the branching/parent/child relation among them is also preserved, as far as I understand.

git log --tags --graph --oneline --decorate --simplify-by-decoration