manifest.walk: join nested if-conditions

manifest.walk: join nested if-conditions

This makes it more closely match the similar condition in
manifestdict.matches().

--- a/mercurial/manifest.py	Wed Apr 08 10:01:31 2015 -0700
+++ b/mercurial/manifest.py	Tue Apr 07 22:35:44 2015 -0700
@@ -225,11 +225,10 @@
         fset = set(match.files())
 
         # avoid the entire walk if we're only looking for specific files
-        if fset and not match.anypats():
-            if util.all(fn in self for fn in fset):
-                for fn in sorted(fset):
-                    yield fn
-                raise StopIteration
+        if fset and not match.anypats() and util.all(fn in self for fn in fset):
+            for fn in sorted(fset):
+                yield fn
+            raise StopIteration
 
         for fn in self:
             if fn in fset: