help: always suggest common-cmds if prefix of cmd

help: always suggest common-cmds if prefix of cmd

If someone runs "git st", the command "git status" is not suggested
because it's not one of the closest levenshtein-neighbour.

Reserve the distance of 0 for common commands where the entered command
is a prefixe, as these are often more likely to be what the user meant.

This way, "git status" is the first suggestion, while a list of possible
typos are still suggested as well.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>