tests/test-issue842

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.

hg init test
cd test
echo foo > a
hg ci -Ama

hg up -r0000
echo bar > a
echo % should issue warning
hg ci -Amb

hg up -r0000
echo stuffy > a
echo % should not issue warning
hg ci -q -Amc

hg up -r0000
echo crap > a
hg branch testing
echo % should not issue warning
hg ci -q -Amd