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.
#!/bin/sh
hg init
echo a > a
hg add a
hg commit -m "1" -d "0 0"
hg status
cp a b
hg copy a b
hg status
hg --debug commit -m "2" -d "0 0"
hg history
hg log a
md5sum .hg/data/b.d
hg cat b > bsum
md5sum bsum
hg cat a > asum
md5sum asum
hg verify