cli: tab complete only full D-Bus paths if there is no prefix

When tab completing connections by path name without any already
typed parts, only suggest full D-Bus paths. The numbers are duplicate
and not preferred.

Before:

    $ nmcli connection show path <TAB>
    Display all 118 possibilities? (y or n)
    1                                            46                                           /org/freedesktop/NetworkManager/Settings/29
    10                                           47                                           /org/freedesktop/NetworkManager/Settings/3
    11                                           48                                           /org/freedesktop/NetworkManager/Settings/30
    12                                           49                                           /org/freedesktop/NetworkManager/Settings/31
    13                                           5                                            /org/freedesktop/NetworkManager/Settings/32

Afterwards:

    $ nmcli connection show path <TAB>
    /org/freedesktop/NetworkManager/Settings/1   /org/freedesktop/NetworkManager/Settings/28  /org/freedesktop/NetworkManager/Settings/46
    ...

and

    $ nmcli connection modify path 4<TAB>
    4   40  41  42  43  44  45  46  47  48  49
7 jobs for th/cli-select-connection in 32 minutes and 54 seconds (queued for 1 second)