store: optimize fncache._load a bit by dirdecoding the contents in one go

For a netbeans clone on Windows 7 x64:

  Before:
    $ hg perffncacheload
    ! wall 0.124000 comb 0.124801 user 0.124801 sys 0.000000 (best of 76)

  After:
    $ hg perffncacheload
    ! wall 0.096000 comb 0.093601 user 0.078001 sys 0.015600 (best of 97)

mercurial/store.py

@@ -336,7 +336,7 b' class fncache(object):'
             # skip nonexistent file
             self.entries = set()
             return
-        self.entries = set(map(decodedir, fp.read().splitlines()))
+        self.entries = set(decodedir(fp.read()).splitlines())
         if '' in self.entries:
             fp.seek(0)
             for n, line in enumerate(fp):