Make repeated field merging consistent

[jcready]

This addresses #319

Setup

const source = { bars: [1],    foos: [{ foo: 1 }] };
const target = { bars: [2, 3], foos: [{ foo: 2 }, { foo: 3 }] };
reflectionMergePartial(SomeMessageType, target, source);

Before PR

Repeated Field	Scalar/Enum	Message
result target array is referentially equal to original target array	❌	✅
result target array is deeply equal to source array	✅	❌
result target array is NOT referentially equal to source array	✅	✅

After PR

Repeated Field	Scalar/Enum	Message
result target array is referentially equal to original target array	✅	✅
result target array is deeply equal to source array	✅	✅
result target array is NOT referentially equal to source array	✅	✅

[timostamm]

Thank you, James. I think it is safe to call this is a bug fix - the different behavior between repeated scalar and repeated message fields was clearly unintentional.

[timostamm]

Released in v2.8.0.