revert: take fast path also when not reverting to '.'

This speeds up 'hg revert -r .^ --all --dry-run' on the Mozilla repo
from 4.081s to 0.826s. Note that 'hg revert -r .^ .' does not get any
faster, since '.' does not make match.always() True.

I can't think of a reason it would break anything, and if it does,
it's clearly not covered by tests.

mercurial/cmdutil.py

@@ -2811,7 +2811,7 b' def revert(ui, repo, ctx, parents, *pats'
 
         wctx = repo[None]
         m = scmutil.match(wctx, pats, opts)
-        if not m.always() or node != parent:
+        if not m.always():
             m.bad = lambda x, y: False
             for abs in repo.walk(m):
                 names[abs] = m.rel(abs), m.exact(abs)
@@ -2840,7 +2840,7 b' def revert(ui, repo, ctx, parents, *pats'
             changes = repo.status(node1=node, match=m,
                                   unknown=True, ignored=True, clean=True)
         else:
-            changes = repo.status(match=m)
+            changes = repo.status(node1=node, match=m)
             for kind in changes:
                 for abs in kind:
                     names[abs] = m.rel(abs), m.exact(abs)