vim-patch:9.1.0814: mapset() may remove unrelated mapping (#30941)

Problem:  mapset() may remove unrelated mapping whose {rhs} matches the
          restored mapping's {lhs}.
Solution: only match by {lhs} when unmapping for mapset() (zeertzjq).

closes: vim/vim#15935

fdf135a052

src/nvim/mapping.h

@@ -19,9 +19,10 @@
 
 /// Used for the first argument of do_map()
 enum {
-  MAPTYPE_MAP     = 0,
-  MAPTYPE_UNMAP   = 1,
-  MAPTYPE_NOREMAP = 2,
+  MAPTYPE_MAP       = 0,
+  MAPTYPE_UNMAP     = 1,
+  MAPTYPE_NOREMAP   = 2,
+  MAPTYPE_UNMAP_LHS = 3,
 };
 
 /// Adjust chars in a language according to 'langmap' option.