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.

debian/changelog

@@ -5,6 +5,9 @@ git-annex (5.20140211) UNRELEASED; urgency=medium
     that have particular metadata.
   * Preferred content expressions can use metadata=field=value
     to limit them to acting on files that have particular metadata.
+  * view: New command that creates and checks out a branch that provides
+    a structured view of selected metadata.
+  * vadd, vrm, vpop: New commands for operating within views.
   * Add progress display for transfers to/from external special remotes.
   * Windows webapp: Can set up box.com, Amazon S3, and rsync.net remotes
   * Windows webapp: Can create repos on removable drives.