Test against the previous version if HEAD === tag

[tomzx]

If a commit has been tagged and is the current HEAD (latest changes), then comparing it against itself is useless.

There are two ways to deal with this:

• Determine that HEAD === tag and then directly output that no changes can be found since the initial and final commits are the same
• Find the previous tag (prior to the given tag) and compare those. For instance, if tag=1.3.0 and HEAD is tag=1.3.0, then we should compare tag=1.2.16 against 1.3.0.

I think the second option makes more sense.