purge: classify as a "working directory management" command in help

It seems to me that `hg purge` is clearly a "working directory
management" command and not a "repository management" command. The
working copy is all that's affected, after all; the repo is not
affected at all.

Differential Revision: https://phab.mercurial-scm.org/D8757

hgext/purge.py

@@ -64,7 +64,7 testedwith = b'ships-with-hg-core'
     ]
     + cmdutil.walkopts,
     _(b'hg purge [OPTION]... [DIR]...'),
-    helpcategory=command.CATEGORY_MAINTENANCE,
+    helpcategory=command.CATEGORY_WORKING_DIRECTORY,
 )
 def purge(ui, repo, *dirs, **opts):
     '''removes files not tracked by Mercurial