irmin-pack: add a migration function to the V2 format

[craigfe]

This contains a subset of the changes from #1015, but uses a different implementation strategy: rather than iterating over all of the objects in the V1 store and adding them individually to the V2 store, we delegate migration to the IO instances (which just shift everything along the appropriate number of bytes). This avoids some unnecessary work.

Other differences:

• due to the new implementation, the migration does not touch the index at all – the Index format has not changed (yet... CC @pascutto);
• the temporary stores are created inside the existing one, to ensure that it's on the same device;
• upgrading a store poisons all instance caches with unlinked file descriptors. Rather than trying to detect this dynamically, I've gone with invalidating cache entries. This means that having open instances during migration is undefined behaviour.

[craigfe]

Aside: if we suspect this migration is very disruptive to users, perhaps we should consider adding some amount of blank header space to the IO files in case a future format change can quickly take advantage of those.

[craigfe]

Needing to extract Filename.quote_command here is quite painful. Perhaps we should: (a) add a dependency (b) use Dune more cleverly (c) spend more time thinking about what quoting needs to be done on Windows.

[samoht]

The progress bar tests seem broken

[craigfe]

Indeed... that's what I get for using this PR as a way to get code onto comanche...

Fixed.

[samoht]

Could you rebase on top of master now that #1072 is merged?

[craigfe]

@samoht: Done.

[samoht]

Thanks! The new code looks indeed cleaner and simpler. Merging now.