[MERGE] make it possible to pass NULL_REVISION as the branch_tip parameter to merge_sort

[Martin Pool]

Martin Pool has voted tweak.
Status is now: Conditionally approved
Comment:
I've done John's tweak for the import statement, and will send this up 
for merge.  I agree about pushing out the use of this marker further.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C47C49CC2.8070506%40canonical.com%3E