Re: Improving merge failure message

> Notable points are:
>
>  - End the messages with "Aborting."; they are given when the three-way
>    merge stops without harming the work tree;
>
>  - Do not give the extra message after unpack_trees() already errored out.
>    This "merging of trees failed" message was primarily for debugging
>    merge-recursive itself, and the end user cannot do much with the object
>    names given in the message anyway.
>
>    But do give it under higher verbosity level, or when it happens during
>    the inner merge (the "recursive" one), as unpack_trees() should not
>    fail for the inner merge under normal conditions.
>
> We could later add instructions on how to recover (i.e. "stash changes
> away or commit on a side branch and retry") instead of the silent
> exit(128) I have down there, and then use Peff's advice.* mechanism to
> squelch it (e.g. "advice.mergeindirtytree"), but they are separate topics.