[m-rev.] diff: fix mdb "level"

[Peter Wang]

On Sun, 27 Sep 2020 16:10:42 +1000 "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:
> This should not need review.
> 
> Zoltan.

> Fix the "level" command in mdb.
> 
> trace/mercury_trace_cmd_browsing.c:
>     The "level" command was documented to reset the current ancestor level
>     to the level of the current event (ancestor level 0) when invoked
>     without arguments, but when invoked that way, it reported an error
>     instead. Fix this.

Hmm, maybe it would be more useful for level to print the current level.

Peter