GROOVY-6397: Fix: Regression for command completers
Using the colon as prefix for both shortcuts and commands revealed a quirk
of the Jline2 Argument Completer.

E.g. Completing ":s a" offered the candidates for all ":save", ":show"
and ":set" commands, due to the commond prefix.

Fixing this in JLine ArgumentCompleter does not seem trivial, so adding a
subclass here which is limited but acts more strictly as we need it.

Added more tests
Mini-feature: Completion for the :alias command,
copy-pasted from the help command
Merge pull request #311 from tkruse/fix_command_completion

Fix command completion