[Nitro] darcs external merger

[Reid Thompson]

last pull bit me with a unresolved conflict -- even though i've made no
changes to my local repo.  Earlier threads noted desire for invocation
of an external merge program -- did a little research and ended up at
the darcs manual .... http://www.darcs.net/manual/bigpage.html#defaults

search page for "Resolution of conflicts"

in
~/.darcs/prefs/defaults

add
ALL --external-merge 'xxdiff -m -O -M %o %1 %a %2'

the manual page lists invocations for several merge tools....
kdiff3, tortoiseMerge, emerge3, etc