Teach bash about completing arguments for git-tag

Teach bash about completing arguments for git-tag

Lately I have been doing a lot of calls to `git tag -d` and also to
`git tag -v`.  In both such cases being able to complete the names
of existing tags saves the fingers some typing effort.  We now look
for the -d or -v option to git-tag in the bash completion support
and offer up existing tag names as possible choices for these.

When creating a new tag we now also offer bash completion support
for the second argument to git-tag (the object to be tagged) as this
can often be a specific existing branch name and is not necessarily
the current HEAD.

If the -f option is being used to recreate an existing tag we now
also offer completion support on the existing tag names for the
first argument of git-tag, helping to the user to reselect the
prior tag name that they are trying to replace.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>