patch.diff: restrict matcher to relative root in certain cases

Previously we'd request all results, then filter by relative root. This is
clearly inefficient, so we now restrict the matcher to the relative root for
certain easy cases.

The particular case here is when the matcher matches all files. In that case we
can simply create a matcher by the relative root.

This is purely an optimization and has no impact on correctness.

mercurial/patch.py

@@ -2105,6 +2105,13 b' 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 b' 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.
+        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)