Improving "abort: can't merge with ancestor"

[Greg Ward]

When "hg merge" finds that there is nothing to merge, it emits the
rather cryptic message

  abort: can't merge with ancestor

I suspect that the set of people who understand that message is
roughly equal to the set of people reading this list.  I.e. end users
are confused by it.  (I sure was the first time I saw it, and I get
asked "what does this mean?" fairly regularly by my users.)

Here's my idea for improving it:

  abort: nothing to merge: merge source (960ab17c3c4e) already an
ancestor of target

Not sure if including the cset id is a good idea.  It makes the
English message 82 chars, which is about... umm... 12 chars too long.
Gee, where can I save 12 chars from that message?  ;-)

Let the bikeshedding commence.  I'll send a patch when/if we achieve
something vaguely like consensus.

Greg