$ example.py diff
show difference between best and last config
$ example.py diff best 1
show difference between best config and next to last
$ example.py diff 5 6
use relative positions, -1 is the first commit in branch
$ example.py diff 20140503.120115 6
use the commit names and mix it with positions
$ example.py diff 20140503.120115 6 -k a b
specify keys in configs to show
$ example.py diff best last 
  --branchA other_branch --branchB default
best is from other_branch, last is from default branch
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