npm / arborist / #774

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Do not remove global packages when updating by name

Adds update.names to this[_explicitRequests] so that when we update a
single named package in the global space, it doesn't read the entire
global actual tree, resulting in a diff which tells reify to remove
everything else.

Fix: https://github.com/npm/cli/issue/2736
Credit: @isaacs
Reviewed-by: @nlf, @ruyadorno

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