* lisp/info.el (Info-file-completions): New variable.
(Info-read-node-name-1): Complete node names in the Info file when a file name is given. Call `Info-build-node-completions' with a file name. (Info-build-node-completions): Add new arg `file'. When it is non-nil, visit it in a temporary buffer and cache its completions in `Info-current-file-completions'. Move most of the function body to `Info-build-node-completions-1'. (Info-build-node-completions-1): New function with the body from `Info-build-node-completions'. Fixes: debbugs:12456
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