tag: add the option to force signing of annotated tags

tag: add the option to force signing of annotated tags

The `tag.forcesignannotated` configuration variable makes "git tag"
that would implicitly create an annotated tag to instead create a
signed tag.  For example

	$ git tag -m "This is a message" tag-with-message
	$ git tag -F message-file tag-with-message

would create a signed tag if the configuration variable is in
effect.  To override this from the command line, the user can
explicitly ask for an annotated tag, like so:

	$ git tag -a -m "This is a message" tag-with-message
	$ git tag -a -F message-file tag-with-message

Creation of a light-weight tag, i.e.

	$ git tag lightweight

is not affected.

Signed-off-by: Laurent Arnoud <laurent@spkdev.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>