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.

Command/PreCommit.hs

@@ -23,6 +23,8 @@ import Logs.View
 import Logs.MetaData
 import Types.View
 import Types.MetaData
+import qualified Git.Index as Git
+import qualified Git.LsFiles as Git
 
 import qualified Data.Set as S
 
@@ -37,10 +39,18 @@ seek ps = lockPreCommitHook $ ifM isDirect
 		withWords startDirect ps
 		runAnnexHook preCommitAnnexHook
 	, do
-		-- fix symlinks to files being committed
-		withFilesToBeCommitted (whenAnnexed Command.Fix.start) ps
-		-- inject unlocked files into the annex
-		withFilesUnlockedToBeCommitted startIndirect ps
+		ifM (liftIO Git.haveFalseIndex)
+			( do
+				(fs, cleanup) <- inRepo $ Git.typeChangedStaged ps
+				whenM (anyM isUnlocked fs) $
+					error "Cannot make a partial commit with unlocked annexed files. You should `git annex add` the files you want to commit, and then run git commit."
+				void $ liftIO cleanup
+			, do
+				-- fix symlinks to files being committed
+				withFilesToBeCommitted (whenAnnexed Command.Fix.start) ps
+				-- inject unlocked files into the annex
+				withFilesUnlockedToBeCommitted startIndirect ps
+			)
 		runAnnexHook preCommitAnnexHook
 		-- committing changes to a view updates metadata
 		mv <- currentView