* lisp/info.el (Info-file-completions): New variable.

* 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