revlog.revision: avoid opening the datafile without need.

If there's no inline data, revlog.revision opens the data file every
time it's called.  This is useful if we're going to call chunk many
times, but, if we're going to call it only once, it's better to let
chunk open the file - if we're lucky, all the data we're going to need
is already cached and we won't need to even look at the file.

mercurial/revlog.py

@@ -933,19 +933,19 b' class revlog(object):'
             raise RevlogError(_('incompatible revision flag %x') %
                               (self.index[rev][0] & 0xFFFF))
 
-        if self._inline:
-            # we probably have the whole chunk cached
             df = None
-        else:
-            df = self.opener(self.datafile)
 
         # do we have useful data cached?
         if self._cache and self._cache[1] >= base and self._cache[1] < rev:
             base = self._cache[1]
             text = str(self._cache[2])
             self._loadindex(base, rev + 1)
+            if not self._inline and rev > base + 1:
+                df = self.opener(self.datafile)
         else:
             self._loadindex(base, rev + 1)
+            if not self._inline and rev > base:
+                df = self.opener(self.datafile)
             text = self.chunk(base, df=df)
 
         bins = [self.chunk(r, df) for r in xrange(base + 1, rev + 1)]