refactor(git): consolidate empty_buf with optional name + bufhidden

lua/git/diff.lua

@@ -110,14 +110,26 @@ function M.git_show_blob(worktree, blob)
     return buf
 end
 
+---@class ow.Git.EmptyBufOpts
+---@field name string?
+---@field bufhidden ("hide"|"wipe")?  defaults to "wipe"
+
+---Build a read-only scratch buffer, optionally naming it via
+---`nvim_buf_set_name` (silently no-op if a buffer with that name
+---already exists).
+---@param opts ow.Git.EmptyBufOpts?
 ---@return integer
-function M.empty_buf()
+function M.empty_buf(opts)
+    opts = opts or {}
     local buf = vim.api.nvim_create_buf(false, true)
     vim.bo[buf].buftype = "nofile"
-    vim.bo[buf].bufhidden = "wipe"
+    vim.bo[buf].bufhidden = opts.bufhidden or "wipe"
     vim.bo[buf].swapfile = false
     vim.bo[buf].modifiable = false
     vim.bo[buf].modified = false
+    if opts.name then
+        pcall(vim.api.nvim_buf_set_name, buf, opts.name)
+    end
     return buf
 end