SI-8981 Tweak REPL help

[som-snytt]

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.

[gkossakowski]

How does the fix work? What's the new message?

[som-snytt]

@gkossakowski Updated the PR conversation. The current message sounds exactly like my seven-year-old.

[lrytz]

LGTM, thanks!