tags: avoid generating commit for an already deleted tag (issue5752)

Authored by navaneeth.suresh.

Description

tags: avoid generating commit for an already deleted tag (issue5752)

$ hg tag --remove <tagname> will keep on generating commits
regardless of checking whether the tag is deleted or not. I added a
a fix for global tags by aborting if nullid is already mapped
to the tag that is to be deleted.

Differential Revision: https://phab.mercurial-scm.org/D5780