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.
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
pulling from ../source2
pulling from ../source1
requesting all changes
adding changesets
adding manifests
adding file changes
added 10 changesets with 10 changes to 1 files
(run 'hg update' to get a working copy)
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
(run 'hg heads' to see heads, 'hg merge' to merge)
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
1 files, 11 changesets, 11 total revisions