subrepo: allow a representation of the working directory subrepo

Some code cannot handle a subrepo based on the working directory (e.g.
sub.dirty()), so the caller must opt in.  This will be useful for archive, and
perhaps some other commands.  The git and svn methods where this is used may
need to be fixed up on a case by case basis.

mercurial/context.py

@@ -249,11 +249,18 b' class basectx(object):'
             return ''
 
     def sub(self, path):
+        '''return a subrepo for the stored revision of path, never wdir()'''
         return subrepo.subrepo(self, path)
 
     def nullsub(self, path, pctx):
         return subrepo.nullsubrepo(self, path, pctx)
 
+    def workingsub(self, path):
+        '''return a subrepo for the stored revision, or wdir if this is a wdir
+        context.
+        '''
+        return subrepo.subrepo(self, path, allowwdir=True)
+
     def match(self, pats=[], include=None, exclude=None, default='glob',
               listsubrepos=False, badfn=None):
         r = self._repo

mercurial/subrepo.py

@@ -324,7 +324,7 b' def _sanitize(ui, vfs, ignore):'
                           "in '%s'\n") % vfs.join(dirname))
                 vfs.unlink(vfs.reljoin(dirname, f))
 
-def subrepo(ctx, path):
+def subrepo(ctx, path, allowwdir=False):
     """return instance of the right subrepo class for subrepo in path"""
     # subrepo inherently violates our import layering rules
     # because it wants to make repo objects from deep inside the stack
@@ -338,6 +338,8 b' def subrepo(ctx, path):'
     state = ctx.substate[path]
     if state[2] not in types:
         raise util.Abort(_('unknown subrepo type %s') % state[2])
+    if allowwdir:
+        state = (state[0], ctx.subrev(path), state[2])
     return types[state[2]](ctx, path, state[:2])
 
 def nullsubrepo(ctx, path, pctx):