Don't allow entering a view with staged or unstaged changes.

In some cases, unstaged changes are safe, eg dotfiles in the top which
are not affected by a view. Or non-annexed files in general which would
prevent view branch checkout from proceeding. But in other cases,
particularly unstaged changes to annexed files, entering a view would wipe
out those changes! And so don't allow entering a view with any unstaged
changes.

Staged changes are not safe when entering a view, because the changes get
committed to the view branch, and so the user is unlikely to remember them
when they exit the view, and so will effectively lose them, even if they're
still present in the view branch.

Also, improved the git status parser, although the improvement turned out
to not really be needed.

This commit was sponsored by Eric Drechsel on Patreon.

Git/Status.hs

@@ -1,6 +1,6 @@
 {- git status interface
  -
- - Copyright 2015 Joey Hess <id@joeyh.name>
+ - Copyright 2015-2018 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -20,6 +20,11 @@ data Status
 	| TypeChanged TopFilePath
 	| Untracked TopFilePath
 
+data StagedUnstaged a = StagedUnstaged
+	{ staged :: Maybe a
+	, unstaged :: Maybe a
+	}
+
 statusChar :: Status -> Char
 statusChar (Modified _) = 'M'
 statusChar (Deleted _) = 'D'
@@ -36,35 +41,32 @@ statusFile (Renamed _oldf newf) = newf
 statusFile (TypeChanged f) = f
 statusFile (Untracked f) = f
 
-parseStatusZ :: [String] -> [Status]
+parseStatusZ :: [String] -> [StagedUnstaged Status]
 parseStatusZ = go []
   where
 	go c [] = reverse c
 	go c (x:xs) = case x of
-		(sindex:sworktree:' ':f) -> 
-			-- Look at both the index and worktree status,
-			-- preferring worktree.
-			case cparse sworktree <|> cparse sindex of
-				Just mks -> go (mks (asTopFilePath f) : c) xs
-				Nothing -> if sindex == 'R'
-					-- In -z mode, the name the
-					-- file was renamed to comes
-					-- first, and the next component
-					-- is the old filename.
-					then case xs of
-						(oldf:xs') -> go (Renamed (asTopFilePath oldf) (asTopFilePath f) : c) xs'
-						_ -> go c []
-					else go c xs
+		(sstaged:sunstaged:' ':f) -> 
+			case (cparse sstaged f xs, cparse sunstaged f xs) of
+				((vstaged, xs1), (vunstaged, xs2)) ->
+					let v = StagedUnstaged
+						{ staged = vstaged
+						, unstaged = vunstaged
+						}
+					    xs' = fromMaybe xs (xs1 <|> xs2)
+					in go (v : c) xs'
 		_ -> go c xs
 
-	cparse 'M' = Just Modified
-	cparse 'A' = Just Added
-	cparse 'D' = Just Deleted
-	cparse 'T' = Just TypeChanged
-	cparse '?' = Just Untracked
-	cparse _ = Nothing
+	cparse 'M' f _ = (Just (Modified (asTopFilePath f)), Nothing)
+	cparse 'A' f _ = (Just (Added (asTopFilePath f)), Nothing)
+	cparse 'D' f _ = (Just (Deleted (asTopFilePath f)), Nothing)
+	cparse 'T' f _ = (Just (TypeChanged (asTopFilePath f)), Nothing)
+	cparse '?' f _ = (Just (Untracked (asTopFilePath f)), Nothing)
+	cparse 'R' f (oldf:xs) =
+		(Just (Renamed (asTopFilePath oldf) (asTopFilePath f)), Just xs)
+	cparse _ _ _ = (Nothing, Nothing)
 
-getStatus :: [CommandParam] -> [FilePath] -> Repo -> IO ([Status], IO Bool)
+getStatus :: [CommandParam] -> [FilePath] -> Repo -> IO ([StagedUnstaged Status], IO Bool)
 getStatus ps fs r = do
 	(ls, cleanup) <- pipeNullSplit ps' r
 	return (parseStatusZ ls, cleanup)