Reduced reduce() usage

[Brad Walker]

​https://github.com/bradmwalker/django/commit/505d9652a8a2ebc9598577ea9da99608432d25d6

[Tim Graham <timograham@…>]

In cfa26f29bdd9b0f82211e6f5ff7f4cd63bd66150:

Reduced reduce() usage; refs #23796.

django.core.exceptions.ValidationError.messages() and
django.db.backends.schema.BaseDatabaseSchemaEditor._alter_field():
Replaced reduce(operator.add, ...) w/uncoupled, explicit sum()

[Tim Graham <timograham@…>]

In 54d3dcbc51819ea0e36bb4dbe7de1a1e33f62c4d:

Removed reduce() usage in makemigrations; refs #23796.

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.