prepush: fix bug in warning message selection
In certain situations you would be told "you should pull and merge" even when
there is nothing to pull.
adding alpha
adding beta
updating to branch default
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
pushing to $HGTMP/test-push-validation/test
searching for changes
adding changesets
adding manifests
adding file changes
transaction abort!
rollback completed
abort: missing file data for beta:dddc47b3ba30e54484720ce0f4f768a0f4b6efb9 - run hg verify