Ruby

Fix subtraction of civar #7045

Job	Run time
C99_compile (3.4)	46s
C99_compile (head)	1m 15s
test (3.1, test)	3m 21s
test (3.3, test)	3m 26s
test (3.4, test, --enable-frozen-string-literal)	3m 12s
test (3.4, stdlib_test)	2m 24s
test (3.4, test)	3m 7s
test (3.4, stdlib_test, --enable-frozen-string-literal)	2m 27s
test (3.2, test)	2m 48s
test (head, stdlib_test rubocop)	3m 25s
test (head, test)	3m 30s
test (3.4, typecheck_test)	1m 12s
test (3.4, rubocop validate test_doc build test_generate_stdlib raap)	2m 47s
	33m 40s

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