Deterministic completion for directories only adds a trailing slash after second Tab press

Suppose I want to `cd` to `.config/gedit`. With usual bash, I do:
```
$ cd .co<TAB>
```
which completes it to
```
$ cd .config/
```
and I can type the rest.

With partial path completion enabled, I do
```
$ cd .co<TAB>
```
which completes it to
```
$ cd .config
```
and then only a second Tab keypress completes it to
```
$ cd .config/
```

Demo: https://asciinema.org/a/549122