[FR] Add option to annotate imported tags with a default message

[dhs-rec]

Currently, all tags imported from Mercurial become s.c. "lightweight" tags in the target Git repository. Commands like git describe ignore such lightweight tags by default, so it would be great if the tags could optionally be annotated, for example

• --annotated-tags creates annotated tags with a default message, like "Imported by hg-fast-export."
• --annotated-tags=<message> creates annotated tags with the given message

[frej]

This could be useful, patches welcome.