fetch: Fix a bug that caused plaintexts to be interpreted as missing more often than they should be. Also refactored a large part of hg_delta_editor to make it carry slightly less state around.

#!/bin/sh
for b in `hg branches | cut -f 1 -d ' '` ; do
    hg co $b || break
    echo Verifying $b
    $(dirname $0)/bisect-find-bad.sh > /dev/null || break
    echo $b Verified.
done