feat(diffthis): retain cursor position from source window

Signed-off-by: Joshua Rubin <me@jawa.dev>

teal/gitsigns/diffthis.tl

@@ -73,31 +73,10 @@ local function run(base: string, diffthis: boolean, vertical: boolean)
   local comp_rev = bcache:get_compare_rev(util.calc_base(base))
   local bufname = bcache:get_rev_bufname(comp_rev)
 
-  if diffthis then
-    -- Enable diff on the currently open buffer
-    vim.cmd'diffthis'
-
-    vim.cmd(table.concat({
-      'keepalt', 'aboveleft',
-      vertical and 'vertical' or '',
-      'split', bufname
-    }, ' '))
-  else
-    -- TODO(lewis6991): Pull in mods from the :Gitsigns command (split, vsplit,
-    -- etc)
-    vim.cmd(table.concat({
-      'edit', bufname
-    }, ' '))
-  end
-
-  local dbuf = vim.api.nvim_get_current_buf()
+   local dbuf = vim.api.nvim_create_buf(false, true)
+   vim.api.nvim_buf_set_name(dbuf, bufname)
 
   local ok, err = pcall(bufread as function, bufnr, dbuf, base, bcache)
-
-  if diffthis then
-    vim.cmd'diffthis'
-  end
-
   if not ok then
     message.error(err as string)
     scheduler()
@@ -133,6 +112,20 @@ local function run(base: string, diffthis: boolean, vertical: boolean)
     vim.bo[dbuf].buftype = 'nowrite'
     vim.bo[dbuf].modifiable = false
   end
+
+  if diffthis then
+    vim.cmd(table.concat({
+      'keepalt', 'aboveleft',
+      vertical and 'vertical' or '',
+      'diffsplit', bufname
+    }, ' '))
+  else
+    -- TODO(lewis6991): Pull in mods from the :Gitsigns command (split, vsplit,
+    -- etc)
+    vim.cmd(table.concat({
+      'edit', bufname
+    }, ' '))
+  end
 end
 
 M.diffthis = void(function(base: string, vertical: boolean)