fix 'convert' with single commit repositories
The attached patch passes the three 'convert' testcases, and also is
able to successfully convert the git.git repository.
#!/bin/sh
mkdir a
cd a
hg init
echo foo > b
hg add b
hg ci -m "b" -d "1000000 0"
chmod -w .hg/store
cd ..
hg clone a b
chmod +w a/.hg/store # let test clean up
cd b
hg verify