[MERGE] log mainline only by default

[Ian Clatworthy]

Talden wrote:

>> I still think it would be nice to have a separate option for "--levels 0",
>> rather than having "0" be something special, but I seem to be the only one
>> of that opinion.
> 
> Well sort of... I'm happy for -n0 to remain but I would like a
> friendly --show-merges alias for it. not only is 0 'special', it's
> pretty non-obvious.

The missing command currently has an --include-merges option (it doesn't yet
support most of the recently added options to log). For consistency, I'd like
to use that name instead of --show-merges.

Otherwise, there's a small risk of redundancy but it's probably worth it in
this case. I'll therefore put a patch up for log --include-merges soon.

Ian C.