refactor(git): promote set_buf_name_and_filetype to diff.M

lua/git/diff.lua

@@ -145,9 +145,13 @@ function M.load_file_buf(abs_path)
     return buf
 end
 
+---Name a scratch buffer with a `git://...` URI and apply the filetype
+---inferred from the inner path segment. The `nvim_buf_set_name` call is
+---wrapped in pcall because a buffer with that name may already exist
+---(E95).
 ---@param buf integer
 ---@param name string
-local function set_buf_name_and_filetype(buf, name)
+function M.set_buf_name_and_filetype(buf, name)
     pcall(vim.api.nvim_buf_set_name, buf, name)
     local ft = vim.filetype.match({ buf = buf })
     if ft then
@@ -181,7 +185,7 @@ function M.split(opts)
     local is_index = opts.ref == ""
     local other = M.git_show_buf(worktree, opts.ref, rel, is_index)
     local label = is_index and "index" or opts.ref
-    set_buf_name_and_filetype(other, "git://" .. label .. "/" .. rel)
+    M.set_buf_name_and_filetype(other, "git://" .. label .. "/" .. rel)
 
     local split_cmd = opts.vertical and "leftabove vertical sbuffer "
         or "leftabove sbuffer "