Fix: target line was not changed

Lines aren't swappped if the origin line is longer than the target.  The
origin line is replaced by the target line, but the target line remains
unchanged.  The cause is passing 1-indexed number of line
`nvim_buf_get_offset`,  but the function accepts a 0-indexed line
number.

lua/substitute/utils.lua

@@ -165,13 +165,13 @@ function utils.compare_regions(origin, target)
   end
 
   local origin_offset = {
-    start = vim.api.nvim_buf_get_offset(0, origin.marks.start.row) + origin.marks.start.col,
-    finish = vim.api.nvim_buf_get_offset(0, origin.marks.finish.row) + origin.marks.finish.col,
+    start = vim.api.nvim_buf_get_offset(0, origin.marks.start.row - 1) + origin.marks.start.col,
+    finish = vim.api.nvim_buf_get_offset(0, origin.marks.finish.row - 1) + origin.marks.finish.col,
   }
 
   local target_offset = {
-    start = vim.api.nvim_buf_get_offset(0, target.marks.start.row) + target.marks.start.col,
-    finish = vim.api.nvim_buf_get_offset(0, target.marks.finish.row) + target.marks.finish.col,
+    start = vim.api.nvim_buf_get_offset(0, target.marks.start.row - 1) + target.marks.start.col,
+    finish = vim.api.nvim_buf_get_offset(0, target.marks.finish.row - 1) + target.marks.finish.col,
   }
 
   --  < if origin comes before target