Make icomplete-tidy-shadowed-file-names less aggressive

Make icomplete-tidy-shadowed-file-names less aggressive

When using this option and editing input, some transient situations
may arise that lead to file-name shadowing, but that shouldn't
necessarily lead to auto-delete behaviour, which will be suprising.

In '/foo/x/bar', if the user deletes the 'x', shadowing occurs, but
probably shouldn't.  So, somewhat like ido-mode, only auto-tidy
shadowed file names if the user is inserting text at end of input.

* lisp/icomplete.el (icomplete-exhibit): Check this-command.
(icomplete-tidy-shadowed-file-names): Tweak docstring.