Enable new diff option linematch (#14537)

Co-authored-by: Lewis Russell <me@lewisr.dev>

src/nvim/buffer_defs.h

@@ -894,6 +894,8 @@ struct diffblock_S {
   diff_T *df_next;
   linenr_T df_lnum[DB_COUNT];           // line number in buffer
   linenr_T df_count[DB_COUNT];          // nr of inserted/changed lines
+  bool is_linematched;  // has the linematch algorithm ran on this diff hunk to divide it into
+                        // smaller diff hunks?
 };
 
 #define SNAP_HELP_IDX   0