templatekw: re-add showtags() to list tags keyword up in online help

Changeset d69a7fc68ad5 removed "showtags()" definition for "tags"
template keyword from "templatekw.py", because "namespaces" puts a
helper function for it into template keyword map automatically. This
works correctly from the point of view of templating functionality.

But on the other hand, it removed "tags" template keyword from "hg
help templates" unexpectedly, because online help text is built before
"namespaces" puts a helper function for "tags" into template keyword
map.

This patch is a kind of backing d69a7fc68ad5 out, but this implements
"showtags()" with newly introduced "shownames()" instead of originally
used "showlist()".

mercurial/templatekw.py

@@ -382,6 +382,13 b' def shownames(namespace, **args):'
     names = ns.names(repo, ctx.node())
     return showlist(ns.templatename, names, plural=namespace, **args)
 
+# don't remove "showtags" definition, even though namespaces will put
+# a helper function for "tags" keyword into "keywords" map automatically,
+# because online help text is built without namespaces initialization
+def showtags(**args):
+    """:tags: List of strings. Any tags associated with the changeset."""
+    return shownames('tags', **args)
+
 # keywords are callables like:
 # fn(repo, ctx, templ, cache, revcache, **args)
 # with:
@@ -420,6 +427,7 b' keywords = {'
     'phaseidx': showphaseidx,
     'rev': showrev,
     'subrepos': showsubrepos,
+    'tags': showtags,
 }
 
 def _showparents(**args):