show-all-if-unmodified doesn't work when partial match contains an escaped character

[Abon B]

In bash 5.0.17 (readline 8.0.004), consider with
'set show-all-if-unmodified on':

$ touch 'test file-1.txt' 'test file-2.txt'
$ ls test<tab>

This expands to 'ls test\ file-'. Then:

$ ls test\ file-<tab>

The partial match is unchanged, so with show-all-if-unmodified on it
should show completions. However, it behaves as if the setting was not
set and needs a second tab press to show completions.

Contrast that with:

$ touch 'test-file-1.txt' 'test-file-2.txt'
$ ls test<tab>
$ ls test-file-<tab>

Which shows matches with a single tab press as expected.

I also tested with literal tabs in the partial matches, which doesn't
work either. I'd guess it has to do with the modification check not
taking into account the escaped characters.