A lambda all_items_equal() replaced a reduce() that was broken for potential
3+-way merges. A reduce(operator.eq, ...) accumulates bools and can't
generically check equality of all items in a sequence:
>>> bool(reduce(operator.eq, [('migrations', '0001_initial')] * 3))
False
The code now counts the number of common ancestors to calculate slice offsets
for the branches. Each branch shares the same number of common ancestors.
The common_ancestor for loop statement had incomplete branch coverage.
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