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.

Types/View.hs

@@ -5,29 +5,31 @@
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
-{-# LANGUAGE CPP #-}
-
 module Types.View where
 
 import Common.Annex
 import Types.MetaData
 import Utility.QuickCheck
+import qualified Git
 
 import qualified Data.Set as S
 
-#ifdef WITH_TDFA
-import Text.Regex.TDFA
-#else
-#endif
+{- A view is a list of fields with filters on their allowed values,
+ - which are applied to files in a parent git branch. -}
+data View = View
+	{ viewParentBranch :: Git.Branch
+	, viewComponents :: [ViewComponent]
+	}
+	deriving (Eq, Show)
 
-{- A view is a list of fields with filters on their allowed values. -}
-type View = [ViewComponent]
+instance Arbitrary View where
+	arbitrary = View <$> pure (Git.Ref "master") <*> arbitrary
 
 data ViewComponent = ViewComponent
 	{ viewField :: MetaField
 	, viewFilter :: ViewFilter
 	}
-	deriving (Show, Eq)
+	deriving (Eq, Show, Read)
 
 instance Arbitrary ViewComponent where
 	arbitrary = ViewComponent <$> arbitrary <*> arbitrary
@@ -38,34 +40,15 @@ type MkFileView = FilePath -> FileView
 
 data ViewFilter
 	= FilterValues (S.Set MetaValue)
-	| FilterGlob Glob
-
-instance Show ViewFilter where
-	show (FilterValues s) = show s
-	show (FilterGlob g) = getGlob g
-
-instance Eq ViewFilter where
-	FilterValues x == FilterValues y = x == y
-	FilterGlob x == FilterGlob y = x == y
-	_ == _ = False
+	| FilterGlob String
+	deriving (Eq, Show, Read)
 
 instance Arbitrary ViewFilter where
 	arbitrary = do
 		size <- arbitrarySizedBoundedIntegral `suchThat` (< 100)
 		FilterValues . S.fromList <$> vector size
 
-#ifdef WITH_TDFA
-data Glob = Glob String Regex
-#else
-data Glob = Glob String
-#endif
-
-instance Eq Glob where
-	a == b = getGlob a == getGlob b
-
-getGlob :: Glob -> String
-#ifdef WITH_TDFA
-getGlob (Glob g _) = g
-#else
-getGlob (Glob g) = g
-#endif
+{- Can a ViewFilter match multiple different MetaValues? -}
+multiValue :: ViewFilter -> Bool
+multiValue (FilterValues s) = S.size s > 1
+multiValue (FilterGlob _) = True