Teach git-tag about showing tag annotations.

Teach git-tag about showing tag annotations.

The <pattern> for -l is now a shell pattern, not a list of grep parameters.
Option -l may be repeated with another <pattern>.

The new -n [<num>] option specifies how many lines from
the annotation are to be printed.
Not specifieing -n or -n 0 will just produce the tag names
Just -n or -n 1 will show the first line of the annotation on
the tag line.
Other valuse for -n will show that number of lines from the annotation.

The exit code used to indicate if any tag was found.
This is changed due to a different implementation.

A good way to test a tag for existence is to use:
git show-ref --quiet --verify refs/tags/$TAGNAME

Signed-off-by: Matthijs Melchior <mmelchior@xs4all.nl>
Signed-off-by: Junio C Hamano <junkio@cox.net>