git-tag-script updates.

git-tag-script updates.

This adds -a (annotate only but not sign) option "A Large Angry
SCM" <gitzilla@gmail.com> sent to the list, after fixing up the
whitespace corruption in the patch, with some of my own fixes.
Namely:

 * A new flag '-a' can be used to create an unsigned tag
   object;

 * The '-f' flag logic did not do the right thing;

 * When creating a signed tag, we did not check for GPG failure
   as we should;

 * Try to use the key for the tagger identity when signing the
   tag.

Signed-off-by: Junio C Hamano <junkio@cox.net>