add git annex view command

(And a vpop command, which is still a bit buggy.)

Still need to do vadd and vrm, though this also adds their documentation.

Currently not very happy with the view log data serialization. I had to
lose the TDFA regexps temporarily, so I can have Read/Show instances of
View. I expect the view log format will change in some incompatable way
later, probably adding last known refs for the parent branch to View
or something like that.

Anyway, it basically works, although it's a bit slow looking up the
metadata. The actual git branch construction is about as fast as it can be
using the current git plumbing.

This commit was sponsored by Peter Hogg.

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