pre-commit: Block partial commit of unlocked annexed file, since that left a typechange staged in index

I had hoped that the git devs could change git's handling of partial
commits to not use a false index file, but seems not.

So, this relies on some git internals to detect that case. The test suite
has a test case added to catch it if changes to git break it.

This commit was sponsored by Paul Tagliamonte.

Git/Index.hs

@@ -11,6 +11,9 @@ import Common
 import Git
 import Utility.Env
 
+indexEnv :: String
+indexEnv = "GIT_INDEX_FILE"
+
 {- Forces git to use the specified index file.
  -
  - Returns an action that will reset back to the default
@@ -25,7 +28,7 @@ override index = do
 	return $ reset res
   where
 	var = "GIT_INDEX_FILE"
-	reset (Just v) = setEnv var v True
+	reset (Just v) = setEnv indexEnv v True
 	reset _ = unsetEnv var
 
 indexFile :: Repo -> FilePath
@@ -34,3 +37,19 @@ indexFile r = localGitDir r </> "index"
 {- Git locks the index by creating this file. -}
 indexFileLock :: Repo -> FilePath
 indexFileLock r = indexFile r ++ ".lock"
+
+{- When the pre-commit hook is run, and git commit has been run with
+ - a file or files specified to commit, rather than committing the staged
+ - index, git provides the pre-commit hook with a "false index file".
+ -
+ - Changes made to this index will influence the commit, but won't
+ - affect the real index file.
+ -
+ - This detects when we're in this situation, using a heiristic, which
+ - might be broken by changes to git. Any use of this should have a test
+ - case to make sure it works.
+ -}
+haveFalseIndex :: IO Bool
+haveFalseIndex = maybe (False) check <$> getEnv indexEnv
+  where
+	check f = "next-index" `isPrefixOf` takeFileName f