store: optimize fncache._write by direncoding the contents in one go

For a netbeans clone on Windows 7 x64:

  Before:
    $ hg perffncachewrite
    ! wall 0.210000 comb 0.218401 user 0.202801 sys 0.015600 (best of 47)

  After:
    $ hg perffncachewrite
    ! wall 0.104000 comb 0.109201 user 0.078000 sys 0.031200 (best of 95)

mercurial/store.py

@@ -348,7 +348,7 b' class fncache(object):'
     def _write(self, files, atomictemp):
         fp = self.opener('fncache', mode='wb', atomictemp=atomictemp)
         if files:
-            fp.write('\n'.join(map(encodedir, files)) + '\n')
+            fp.write(encodedir('\n'.join(files) + '\n'))
         fp.close()
         self._dirty = False