unlock: Don't allow unlocking files that have never been committed to git before

Avoids an intractable problem that prevents the pre-commit hook from
telling if such a file is intended to be an annexed file or not.

Command/Unlock.hs

@@ -10,6 +10,7 @@ module Command.Unlock where
 import Common.Annex
 import Command
 import Annex.Content
+import Annex.CatFile
 import Utility.CopyFile
 
 cmd :: [Command]
@@ -29,10 +30,10 @@ start :: FilePath -> Key -> CommandStart
 start file key = do
 	showStart "unlock" file
 	ifM (inAnnex key)
-		( ifM (checkDiskSpace Nothing key 0)
+		( ifM (isJust <$> catKeyFileHEAD file)
 			( next $ perform file key
 			, do
-				warning "not enough disk space to copy file"
+				warning "this has not yet been committed to git; cannot unlock it"
 				next $ next $ return False
 			)
 		, do
@@ -41,19 +42,24 @@ start file key = do
 		)
 
 perform :: FilePath -> Key -> CommandPerform
-perform dest key = do
-	src <- calcRepo $ gitAnnexLocation key
-	tmpdest <- fromRepo $ gitAnnexTmpObjectLocation key
-	liftIO $ createDirectoryIfMissing True (parentDir tmpdest)
-	showAction "copying"
-	ifM (liftIO $ copyFileExternal CopyAllMetaData src tmpdest)
-		( do
-			liftIO $ do
-				removeFile dest
-				moveFile tmpdest dest
-			thawContent dest
-			next $ return True
-		, do
-			warning "copy failed!"
-			next $ return False
-		)
+perform dest key = ifM (checkDiskSpace Nothing key 0)
+	( do
+		src <- calcRepo $ gitAnnexLocation key
+		tmpdest <- fromRepo $ gitAnnexTmpObjectLocation key
+		liftIO $ createDirectoryIfMissing True (parentDir tmpdest)
+		showAction "copying"
+		ifM (liftIO $ copyFileExternal CopyAllMetaData src tmpdest)
+			( do
+				liftIO $ do
+					removeFile dest
+					moveFile tmpdest dest
+				thawContent dest
+				next $ return True
+			, do
+				warning "copy failed!"
+				next $ return False
+			)
+	, do
+		warning "not enough disk space to copy file"
+		next $ return False
+	)