help: enable listing of a subset of the command list
Instead of giving the "command %s is ambiguous" error message
with a bare list of possible command, display help list
for all commands starting with the given string.
# should print nothing
# should fail
abort: path 'b/x' is inside repo 'b'
abort: path 'b/x' is inside repo 'b'
# should fail
abort: path 'b/x' is inside repo 'b'
# should arguably print nothing
# should fail
abort: path 'b/a' is inside repo 'b'