Allow the changegroup generator to completely load the buffer.

This makes is possible to delete or truncate away revisions and still
use the changegroup later.

mercurial/hg.py

@@ -1427,6 +1427,9 b' class localrepository:'
             def __init__(self, generator):
                 self.g = generator
                 self.buf = ""
+            def fillbuf(self):
+                self.buf += "".join(self.g)
+
             def read(self, l):
                 while l > len(self.buf):
                     try: