revset: avoid loop for "match.files()" having always one element for efficiency

This patch avoids the loop for "match.files()" having always one
element in revset predicate "filelog()" for efficiency: "match" object
"m" is constructed with "[pat]" as "patterns" argument.

mercurial/revset.py

@@ -722,7 +722,7 def filelog(repo, subset, x):
     s = set()
 
     if not matchmod.patkind(pat):
-        for f in m.files():
+        f = m.files()[0]
             fl = repo.file(f)
             for fr in fl:
                 s.add(fl.linkrev(fr))