[MERGE] show merge indicators in log --long

[Matt Nordhoff]

Ian Clatworthy wrote:
> Also, if only one level is displayed and merges are found,
> the user is now advised how to see the nested merged
> revisions for --long and --short. (I decided against
> adding that advice in long --line mainly because that might
> break scripts parsing the output from there.)
> 
> Ian C.

I may be misunderstanding when merge indicators are usually used, this
is probably too complicated, and it shouldn't block this patch anyway,
but I just want to throw it out there:

Why only show the indicators when there's one level? If it's showing two
levels, but the log actually contains three levels of merges, the user
will be in the exact same situation as today.
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