Add a global variable, 'present_path', so that 'cwd_tab_completion()'
knows where the user is in the browser, so that it can try completions
against names in that directory instead of always against names in the
current working directory (where nano was invoked).

This fixes https://savannah.gnu.org/bugs/?47234

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Signed-off-by: Rishabh Dave <rishabhddave@gmail.com>
Signed-off-by: Benno Schulenberg <bensberg@justemail.net>