Add support for selectively choosing deduplication keys. #195

Job	Run time
test (2.3, rails-5.2.gemfile)	1m 18s
test (2.4, rails-5.2.gemfile)	1m 20s
test (2.5, rails-5.2.gemfile)	1m 0s
test (2.5, rails-6.0.gemfile)	43s
test (2.5, rails-6.1.gemfile)	56s
test (2.6, rails-5.2.gemfile)	55s
test (2.6, rails-6.0.gemfile)	1m 7s
test (2.6, rails-6.1.gemfile)	1m 6s
test (2.7, rails-5.2.gemfile)	1m 7s
test (2.7, rails-6.0.gemfile)	1m 4s
test (2.7, rails-6.1.gemfile)	56s
test (2.7, rails-7.0.gemfile)	54s
test (3.0, rails-6.0.gemfile)	26s
test (3.0, rails-6.1.gemfile)	48s
test (3.0, rails-7.0.gemfile)	26s
test (3.1, rails-6.0.gemfile)	34s
test (3.1, rails-6.1.gemfile)	24s
test (3.1, rails-7.0.gemfile)	42s
test (3.1, rails-main.gemfile)	1m 0s
test (3.2, rails-6.0.gemfile)	26s
test (3.2, rails-6.1.gemfile)	30s
test (3.2, rails-7.0.gemfile)	26s
test (3.2, rails-main.gemfile)	48s
test (jruby-9.1, rails-5.2.gemfile)	1m 31s
test (jruby-9.2, rails-5.2.gemfile)	1m 24s
test (jruby-9.2, rails-6.0.gemfile)	1m 21s
test (jruby-9.2, rails-6.1.gemfile)	1m 42s
test (jruby-9.3, rails-5.2.gemfile)	1m 13s
test (jruby-9.3, rails-6.0.gemfile)	1m 21s
test (jruby-9.3, rails-6.1.gemfile)	1m 30s
test (jruby-9.4, rails-6.0.gemfile)	1m 14s
test (jruby-9.4, rails-6.1.gemfile)	1m 3s
test (jruby-9.4, rails-7.0.gemfile)	58s
test (jruby-9.4, rails-main.gemfile)	1m 54s
	34m 7s

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