fix bug found by unit test: when deleting leftwards, we should never delete beyond the end of the line

fcore/normal-mode/make-normal-delete.sml

@@ -213,7 +213,7 @@ struct
 
   fun deleteByDfa (app: app_type, count, fMove, time) =
     let
-      val {buffer, cursorIdx, searchString, ...} = app
+      val {buffer, cursorIdx, ...} = app
 
       val buffer = LineGap.goToIdx (cursorIdx, buffer)
       val otherIdx = fMove (buffer, cursorIdx, count)
@@ -225,6 +225,20 @@ struct
       deleteAndFinish (app, low, length, buffer, time)
     end
 
+  fun deleteCharsLeft (app: app_type, count, time) =
+    let
+      val {buffer, cursorIdx, ...} = app
+
+      val buffer = LineGap.goToIdx (cursorIdx, buffer)
+      val startOfLine = Cursor.vi0 (buffer, cursorIdx)
+      val low = Cursor.viH (buffer, cursorIdx, count)
+      val low = if low < startOfLine then startOfLine else low
+
+      val length = cursorIdx - low
+    in
+      deleteAndFinish (app, low, length, buffer, time)
+    end
+
   fun deleteToEndOfPrevWord (app: app_type, count, time) =
     let
       val {buffer, cursorIdx, searchString, ...} = app

fcore/normal-mode/normal-mode.sml

@@ -250,7 +250,7 @@ struct
     fun parseDeleteTerminal (str, count, app, chrCmd, time) =
       case chrCmd of
       (* terminal commands: require no input after *)
-        #"h" => NormalDelete.deleteByDfa (app, count, Cursor.viH, time)
+        #"h" => NormalDelete.deleteCharsLeft (app, count, time)
       | #"l" => NormalDelete.deleteByDfa (app, count, Cursor.viL, time)
       (* vi's 'j' and 'k' commands move up or down a column
        * but 'dj' or 'dk' delete whole lines