SI-8981 Tweak REPL help

Tweak colon command processing.

Fixes an unhelpful message about the ambiguity of colon.

Currently,
```
scala> :
 is ambiguous: did you mean :edit or :help or :history or :h? or :imports or :implicits or :javap or :line or :load or :paste or :power or :quit or :replay or :reset or :save or :sh or :settings or :silent or :type or :kind or :warnings?
```
After the fix, just show the help summary.