add pretag and tag hooks.

pretag hook lets hook decide how tags can be named.  tag hook is
notifier.

doc/hgrc.5.txt

@@ -159,6 +159,15 b' hooks::'
   precommit;;
     Run before starting a commit.  Exit status 0 allows the commit to
     proceed.  Non-zero status will cause the commit to fail.
+  pretag;;
+    Run before creating a tag.  Exit status 0 allows the tag to be
+    created.  Non-zero status will cause the tag to fail.  ID of
+    changeset to tag in $NODE.  Name of tag in $TAG.  Tag is local if
+    $LOCAL=1, in repo if $LOCAL=0.
+  tag;;
+    Run after a tag is created.  ID of tagged changeset in $NODE.
+    Name of tag in $TAG.  Tag is local if $LOCAL=1, in repo if
+    $LOCAL=0.
 
 http_proxy::
   Used to access web-based Mercurial repositories through a HTTP

mercurial/commands.py

@@ -2113,8 +2113,12 b' def tag(ui, repo, name, rev_=None, **opt'
         if name.find(c) >= 0:
             raise util.Abort(_("%s cannot be used in a tag name") % repr(c))
 
+    repo.hook('pretag', throw=True, node=r, tag=name,
+              local=not not opts['local'])
+
     if opts['local']:
         repo.opener("localtags", "a").write("%s %s\n" % (r, name))
+        repo.hook('tag', node=r, tag=name, local=1)
         return
 
     for x in repo.changes():
@@ -2130,6 +2134,7 b' def tag(ui, repo, name, rev_=None, **opt'
                _("Added tag %s for changeset %s") % (name, r))
     try:
         repo.commit([".hgtags"], message, opts['user'], opts['date'])
+        repo.hook('tag', node=r, tag=name, local=0)
     except ValueError, inst:
         raise util.Abort(str(inst))