Don't insert prefix for non-prefix matches

The issue here is that when inserting a common prefix for e.g. a
substring match, we increase the amount of available candidates again to
things the user didn't want.

An example is in share/functions - a completion for "inter" would
previously expand to "__fish_" because it matched:

- __fish_config_interactive.fish
- __fish_print_interfaces.fish
- __fish_print_lpr_printers.fish

The completion afterwards would then show 189 possible matches, only
three of which (the above) actually matched the original "inter".

Fixes #3089.

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