Fix #5313 (and continuation of #5240)

The root cause is field splitting by bash.
It is erroneously performed when processing compgen output.
The delimiters used for field splitting are defined in IFS environment variable, which is $' \t\n' (space, tab, newline) by default. If the output of compgen contains these characters, they are split out.

This patch (partially) fixes that by restricting IFS to newline.
Now we can handle filenames with spaces and/or tabs, but not newlines because we cannot distinguish "newline as a delimiter" from "newline contained in filename".

Handling newlines would be more difficult than handling spaces. It seems that other completion scripts cannot handle this.
I think this should be a separate issue if we really try to fix this.