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.
% no working directory
adding a
adding b
% hg parents
changeset: 2:6cfac479f009
tag: tip
user: test
date: Thu Jan 01 00:00:02 1970 +0000
summary: b
% hg parents a
changeset: 0:b6a1406d8886
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: ab
% hg parents -r 2
changeset: 1:d786049f033a
user: test
date: Thu Jan 01 00:00:01 1970 +0000
summary: a
% hg parents -r 2 a
changeset: 0:b6a1406d8886
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: ab