revlog: always open revlogs for reading and appending

An upcoming patch will teach revlogs to use the existing file
handle to read revision data instead of opening a new file handle
just for quick reads. For this to work, files must be opened for
reading as well.

This patch is merely cosmetic: there are no behavior changes.

mercurial/revlog.py

@@ -1218,7 +1218,7 b' class revlog(object):'
 
         dfh = None
         if not self._inline:
-            dfh = self.opener(self.datafile, "a")
+            dfh = self.opener(self.datafile, "a+")
         ifh = self.opener(self.indexfile, "a+")
         try:
             return self._addrevision(node, text, transaction, link, p1, p2,
@@ -1467,7 +1467,7 b' class revlog(object):'
         else:
             transaction.add(self.indexfile, isize, r)
             transaction.add(self.datafile, end)
-            dfh = self.opener(self.datafile, "a")
+            dfh = self.opener(self.datafile, "a+")
         def flush():
             if dfh:
                 dfh.flush()
@@ -1535,8 +1535,8 b' class revlog(object):'
                     # addrevision switched from inline to conventional
                     # reopen the index
                     ifh.close()
-                    dfh = self.opener(self.datafile, "a")
-                    ifh = self.opener(self.indexfile, "a")
+                    dfh = self.opener(self.datafile, "a+")
+                    ifh = self.opener(self.indexfile, "a+")
         finally:
             if dfh:
                 dfh.close()