In the bash shell we used to print ActiveHelp messages on every
tab-press. In the example below, notice the "Command help" line which is
ActiveHelp:

bash-5.1$ tanzu context u[tab]
Command help: Configure and manage contexts for the Tanzu CLI

bash-5.1$ tanzu context u[tab]
Command help: Configure and manage contexts for the Tanzu CLI

bash-5.1$ tanzu context u
unset  (Unset the active context so that it is not used by default.)
use    (Set the context to be used by default)
bash-5.1$ tanzu context u

Above, on the first [tab] press, only the ActiveHelp is printed.
On the second [tab] press, the ActiveHelp is printed again, followed
by a re-print of the command-line, followed by the completions choices.

The separation between ActiveHelp and completion choices makes the
ActiveHelp harder to see. Furthermore, I find the double printing of the
ActiveHelp string to look bad.

Note that for zsh, the UX is different and that ActiveHelp messages are
printed at the same time as the completion choices.

This commit aligns the UX for ActiveHelp in bash with the one for zsh:
if there are other completions to be shown, the ActiveHelp messages are
printed at the same time.

New behaviour:
1- ActiveHelp is no longer printed on the first [tab] press. This is
   better aligned with bash's standard approach.
2- ActiveHelp is printed on the second [tab] press, above the completion
   choices, with a `--` delimiter.
3- If there are no completion choices, the `--` delimiter is omitted.

This behaviour is the same as what is done for zsh (except that for zsh
the first [tab] press immediately shows completion choices).

Below is the above example, but using this commit.
Notice the more concise and easier to read completion output:

bash-5.1$ tanzu context u[tab][tab]
Command help: Configure and manage contexts for the Tanzu CLI
--
unset  (Unset the active context so that it is not used by default.)
use    (Set the context to be used by default)
bash-5.1$ tanzu context u

Signed-off-by: Marc Khouzam <[email protected]>