cindex: switch --join to use dfpost7 by default

Post-image blob OIDs are what solver already works with, and
longer OIDs may not be available in historical mail archives.

`patchid' turns out to be unsuitable since:
1) git's default diff algorithm has changed over time
2) users may use different diff options to improve readability

Of course, we could eventually run `lei rediff' during the index
phase to regenerate patchids, but that's out-of-scope for now
and likely to be too expensive.

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