sh: improve command completion

When there are many matches, find the longest common substring starting
from the beginning of each command and use that to replace input.

As an example: on my system, llv<tab> will be autocompleted to llvm-
and another <tab> will print all matching llvm commands.

(cherry picked from commit c866d0c798a20b8f0a92df524f4ddd0d81511c88)