Fix a bug in hidden field handling in std.mergePatch

[JoshRosen]

This PR fixes a bug related to handling of hidden fields in std.mergePatch:

In both google/jsonnet and google/go-jsonnet, hidden fields are ineligible to participate in field merges:

• std.mergePatch({a: 1}, {a :: 2}
  • should return {a: 1} (ignoring the hidden patch field)
  • sjsonnet returns {a: 2}
• std.mergePatch({a: {a: 1}}, {a:: {b: 1}}
  • should return {a: {a: 1}}
  • sjsonnet returns {a: {a: 1, b: 1}}
• std.mergePatch({a:: {a: 1}}, {a: {b: 1}} (symmetrical to previous case, but swaps hidden between target and patch)
  • should return {a: {b: 1}}
  • sjsonnet returns {a: {a: 1, b: 1}}

The problem is that the current code iterates over the union of visible fields, but never re-checks the specific visibility in target and patch during per-field merges.

This PR fixes that bug and significantly expands unit test coverage for std.mergePatch.