diff --git a/mercurial/patch.py b/mercurial/patch.py
--- a/mercurial/patch.py
+++ b/mercurial/patch.py
@@ -2105,6 +2105,13 @@ def diff(repo, node1=None, node2=None, m
     ctx1 = repo[node1]
     ctx2 = repo[node2]
 
+    relfiltered = False
+    if relroot != '' and match.always():
+        # as a special case, create a new matcher with just the relroot
+        pats = [relroot]
+        match = scmutil.match(ctx2, pats, default='path')
+        relfiltered = True
+
     if not changes:
         changes = repo.status(ctx1, ctx2, match=match)
     modified, added, removed = changes[:3]
@@ -2123,14 +2130,16 @@ def diff(repo, node1=None, node2=None, m
         copy = copies.pathcopies(ctx1, ctx2)
 
     if relroot is not None:
-        # XXX this would ideally be done in the matcher, but that is generally
-        # meant to 'or' patterns, not 'and' them. In this case we need to 'and'
-        # all the patterns from the matcher with relroot.
-        def filterrel(l):
-            return [f for f in l if f.startswith(relroot)]
-        modified = filterrel(modified)
-        added = filterrel(added)
-        removed = filterrel(removed)
+        if not relfiltered:
+            # XXX this would ideally be done in the matcher, but that is
+            # generally meant to 'or' patterns, not 'and' them. In this case we
+            # need to 'and' all the patterns from the matcher with relroot.
+            def filterrel(l):
+                return [f for f in l if f.startswith(relroot)]
+            modified = filterrel(modified)
+            added = filterrel(added)
+            removed = filterrel(removed)
+            relfiltered = True
         # filter out copies where either side isn't inside the relative root
         copy = dict(((dst, src) for (dst, src) in copy.iteritems()
                      if dst.startswith(relroot)