RFR 8142447: Command change: re-run n-th command should be re-run by id

[ShinyaYoshida]

Hi Robert,
I propose changing /<n> to /<id>.

There is no one-to-one correspondence between n-th snippet and snippet id
because we have the name space for main, startup and error.
(Note: When the re-run command is implemented, we have just only one name
space for any snippet)

It makes re-run difficult.
When we re-run main snippet, we have to add the number of startup snippets
and the number of erroneous snippets which appear prior to the snippet what
we want to re-run.
It is very confusing for users.

You can see the example at bugs:
https://bugs.openjdk.java.net/browse/JDK-8142447

Here is the patch.
Webrev: http://cr.openjdk.java.net/~shinyafox/kulla/8142447/webrev.00/

Could you consider and review this?

Regards,
shinyafox(Shinya Yoshida)