store: replace invocation of "getsize()" by "vfs.stat()"

This patch replaces invocation of "getsize()", which calls "os.stat()"
internally, by "vfs.stat()".

The object referred by "self.rawvfs" is used internally by
"_fncachevfs" and doesn't encode filename for each file API invocation.

This patch invokes "os.stat()" via "self.rawvfs" to avoid redundant
filename encoding: invocation of "os.stat()" via "self.vfs" hides
filename encoding and encoding result from caller, so it is not
appropriate, when both encoded and non-encoded filenames should be
yield.

Even though changeset b42b0729744d improved stream_out performance by
"self.pathsep + path", this patch replaces it by
"os.path.join(self.base, path)" of vfs. So, this may increase cost to
join path components.

But this shouldn't have large impact, because:

  - such cost is much less than cost of "os.stat()" which causes
    system call invocation

  - "datafiles()" of store object is invoked only for "hg manifest
    --all" or "hg verify" which are both heavy functions

mercurial/store.py

@@ -459,6 +459,7 b' class fncachestore(basicstore):'
         self.pathsep = self.path + '/'
         self.createmode = _calcmode(vfs)
         vfs.createmode = self.createmode
+        self.rawvfs = vfs
         fnc = fncache(vfs)
         self.fncache = fnc
         self.vfs = _fncachevfs(vfs, fnc, encode)
@@ -467,16 +468,14 b' class fncachestore(basicstore):'
     def join(self, f):
         return self.pathsep + self.encode(f)
 
-    def getsize(self, path):
-        return os.stat(self.pathsep + path).st_size
-
     def datafiles(self):
         rewrite = False
         existing = []
+        getstat = self.rawvfs.stat
         for f in sorted(self.fncache):
             ef = self.encode(f)
             try:
-                yield f, ef, self.getsize(ef)
+                yield f, ef, getstat(ef).st_size
                 existing.append(f)
             except OSError, err:
                 if err.errno != errno.ENOENT: