Portability + log on HEAD detection

Hello,

Please find a portability fix for this module. Depends on IPC::Cmd that is a core module.

In addition:
- Since your module works on tags it should IMHO use refs/tags/$tag in all places where a tag is the input
- The detection of an eventual HEAD is wrong, it will compare a an hex commit value with a tag

Regards, Jean-Damien.