hg-fast-export: Compare HG revisions when merging
hg-fast-export uses hg's branch order (from the log) when merging,
this is a problem. Consider the case:
HG repo A has revisions 1-10. Repository B is cloned from that.
Subsequently, A adds revision 11, and B adds a different change which
also has revision 11. If B now pulls from A, A's rev11 will have the
number 12; if A then pulls from B, the reverse also holds. So the logs
are different even though they contain the exact same changes.
hg-fast-export will thus create different git repositories for A and B,
even though the contents are identical for all practical purposes.
In particular, the repos would be identical if A and B had used git from
the beginning.
To fix that, compare HG revisions instead of log positions.