store: speed up read and write of large fncache files

In my tests of an fncache containing 300,000 entries, this improves
read time from 567ms to 307, and write time from 1328ms to 533.

These numbers aren't so great, since the fncache file is only 17MB
in size, but they're an improvement.

mercurial/store.py

@@ -317,36 +317,36 b' class fncache(object):'
 
     def _load(self):
         '''fill the entries from the fncache file'''
-        self.entries = set()
         self._dirty = False
         try:
             fp = self.opener('fncache', mode='rb')
         except IOError:
             # skip nonexistent file
+            self.entries = set()
             return
+        self.entries = set(map(decodedir, fp.read().splitlines()))
+        if '' in self.entries:
+            fp.seek(0)
         for n, line in enumerate(fp):
-            if (len(line) < 2) or (line[-1] != '\n'):
+                if not line.rstrip('\n'):
                 t = _('invalid entry in fncache, line %s') % (n + 1)
                 raise util.Abort(t)
-            self.entries.add(decodedir(line[:-1]))
         fp.close()
 
+    def _write(self, files, atomictemp):
+        fp = self.opener('fncache', mode='wb', atomictemp=atomictemp)
+        if files:
+            fp.write('\n'.join(map(encodedir, files)) + '\n')
+        fp.close()
+        self._dirty = False
+
     def rewrite(self, files):
-        fp = self.opener('fncache', mode='wb')
-        for p in files:
-            fp.write(encodedir(p) + '\n')
-        fp.close()
+        self._write(files, False)
         self.entries = set(files)
-        self._dirty = False
 
     def write(self):
-        if not self._dirty:
-            return
-        fp = self.opener('fncache', mode='wb', atomictemp=True)
-        for p in self.entries:
-            fp.write(encodedir(p) + '\n')
-        fp.close()
-        self._dirty = False
+        if self._dirty:
+            self._write(self.entries, True)
 
     def add(self, fn):
         if self.entries is None: