context: extend efficient manifest filtering to when all paths are files

On a repository with over 250,000 files and 700,000 commits, this improves
cases like

hg status --rev <rev> -- <file>  # rev is not .

from 2.1 seconds to 1.4 seconds.

There is further scope for improvement here: for a single file or a small set
of files, it is probably more efficient to use filelog linkrevs when possible.
However there will always be cases where that will fail (multiple commits
pointing to the same file revision, removed files...), so this is independently
useful.

mercurial/context.py

@@ -74,8 +74,10 b' class basectx(object):'
         if match.always():
             return self.manifest().copy()
 
-        if match.matchfn == match.exact:
-            return self.manifest().intersectfiles(match.files())
+        files = match.files()
+        if (match.matchfn == match.exact or
+            (not match.anypats() and util.all(fn in self for fn in files))):
+            return self.manifest().intersectfiles(files)
 
         mf = self.manifest().copy()
         for fn in mf.keys():