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.
adding a
adding b
copy .hg/patches to .hg/patches.1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
M b
a
b
merging with queue at: .hg/patches.1
applying rm_a
Now at: rm_a
b
Patch queue now empty