Windows: `startify_change_to_dir` not applied if filename has spaces

On Windows, if the filename has spaces, Startify fails to change the directory.

Somewhere along the line, `a:path`, the path to the file that was selected, gets its spaces escaped with backslashes and this seems to be the cause of the issue.

On line 973 of autoload/startify, it calls `fnamemodify(a:path, ':h')` but only escapes up to the last space of the filename (e.g. `C:\Users\rioog\folder\file\ with\ spaces` becomes `C:\Users\rioog\folder\file\ with`). As this is not a valid directory name, it does not change the directory.

I haven't been able to track down where this improper escaping takes place so for now I've just changed that line to `fnamemodify(expand('%'), ':h')`.

Start in autoload/startify around line 973 and inspect how startify_change_to_dir passes the selected a:path to fnamemodify. Reproduce the Windows case with a filename containing spaces and verify that the directory changes to the file's parent without truncating the path.

Written by the indexing model from the issue text.