allow / handle file-names with spaces about comparewith HOT 1 CLOSED

To address this, one can probably rely on shQuote(), similar to what system2() already uses internally for the command (see help(system2)).

At the same time, we should also avoid passing filter = "*.*" to rstudioapi::selectFile() and live with the default, which does not restrict to files with extension. Otherwise, we would be excluding e.g. NAMESPACE and DESCRIPTION files.

As part of this, we should also move to compare_meld() the normalizePath(), possibly wrapping shQuote(normalizePath()) into a small utility.

from comparewith.