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

[kolayne]

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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

[sio]

Original PR #24 had been left unfinished by author, this issue welcomes help from new contributors!