functions: speed up compgroup determination

[yannk]

We could also limit the number of tests so that it is not slow.

Fix #26

[gazorby]

Hi @yannk, thanks for this PR!

I installed your fork locally and completing ls fails:

~/.config/fish/functions/_fifc_completion_group.fish (line 1): Unexpected end of string, quotes are not balanced

[gazorby]

This should be fixed now in v0.1.1, thanks again!