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.
adding a
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
a renamed from b:9a263dd772e0159ddfa70fb8a448bb6c7ec8c69f
% test with --rev
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3