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
# test parents command
hg init a
cd a
echo % no working directory
hg parents
echo a > a
echo b > b
hg ci -Amab -d '0 0'
echo a >> a
hg ci -Ama -d '1 0'
echo b >> b
hg ci -Amb -d '2 0'
echo % hg parents
hg parents
echo % hg parents a
hg parents a
echo % hg parents -r 2
hg parents -r 2
echo % hg parents -r 2 a
hg parents -r 2 a