ash: perform autocompletion on builtin commands

Refactor read_line_input to accept a list of strings which are
added byt the autocompletion suggestions.
Let ash pass the builtin command list to read_line_input to enable
completion of builtin commands as echo, printf, history, exec, etc.
gitignore: add testsuite/echo-ne

It shows up in git status as untracked.

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>