pick 252a1af424ad 2 Blorb a morgwazzle
         pick 5339bf82f0ca 3 Zworgle the foobar
         pick 8ef592ce7cc4 4 Bedazzle the zerlog

       To swap revision 2 and 4, its lines are swapped
       in the editor::

         pick 8ef592ce7cc4 4 Bedazzle the zerlog
         pick 5339bf82f0ca 3 Zworgle the foobar
         pick 252a1af424ad 2 Blorb a morgwazzle

Returns 0 on success, 1 if user intervention is required (not only
for intentional "edit" command, but also for resolving unexpected
conflicts).
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