Git force signing of annotated tags

The patch is included in the next minor release of Git 2.9.0.

It was really interesting to contribute to Git. Difficult to understand all implication of the command line options, but I really liked the old fashioned review and that the project has really good integration tests.

Of course, it is not my every day work to code in C, so I would like to thank Junio and Jeff for their patience and help.