Fix merged source name generation when shared root is operation root

[AnthonyMDev]

This fixes #3071

When the shared root of a merged source is the direct parent or a direct sibling, we remove the parent from the namespacing of the referenced selection set. If the shared root is the root of the operation/fragment, that component of the namespacing should be removed.

Since the change to the way we indicate the location of a SelectionSet in the IR changed in #3045, in this case, the shared root is now not part of the fieldPath, causing the "shared root" to be at an index of -1 (because its represented by the location.source now). This was causing a crash with an invalid range bounds.

This PR fixes this by using max(0, sharedRootIndex) to ensure we don't use -1, and then not removing the first component in that case (because that component would be the location.source which is already not included.

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[calvincestari]

Thanks @AnthonyMDev 🎉