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.
+ hg init
+ hg add a
+ hg commit -m a -u a -d 0 0
+ hg init
+ hg add b
+ hg commit -m b -u b -d 0 0
+ hg pull ../a
pulling from ../a
searching for changes
warning: pulling from an unrelated repository!
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
(run 'hg update' to get a working copy)
+ hg heads
changeset: 1:9a79c33a9db3
tag: tip
user: a
date: Thu Jan 1 00:00:00 1970
summary: a
changeset: 0:01f8062b2de5
user: b
date: Thu Jan 1 00:00:00 1970
summary: b