refactor(git): scratch buffers wipe by default, consolidate factories in util

lua/git/object.lua

@@ -229,10 +229,7 @@ end
 local function blob_buf(worktree, blob, path, ref)
     local revspec = ref .. ":" .. path
     if is_zero(blob) then
-        return diff.empty_buf({
-            name = util.uri(revspec),
-            bufhidden = "hide",
-        })
+        return util.empty_buf({ name = util.uri(revspec) })
     end
     return M.buf_for(worktree, revspec)
 end
@@ -269,10 +266,7 @@ end
 ---@field split (false|"above"|"below"|"left"|"right")?  forwarded to `util.new_scratch`. Default opens a new horizontal split.
 
 ---Place a `git://<revspec>` URI buffer in a window per `opts.split`.
----`bufadd` dedups against existing buffers, so re-opening the same URI
----reuses the buffer (and `bufload` no-ops). `read_uri` defaults to
----`bufhidden=wipe` (right for diff sides), but navigation buffers
----should persist across window closes, so override to `hide`.
+---`bufadd` dedups against existing buffers; `bufload` no-ops if loaded.
 ---@param worktree string
 ---@param uri string
 ---@param sha string  written to `b:git_ref` so `<CR>` navigation in the buffer can resolve relative paths
@@ -283,7 +277,6 @@ local function open_uri(worktree, uri, sha, opts, default_ft)
     vim.b[buf].git_worktree = worktree
     vim.b[buf].git_ref = sha
     vim.fn.bufload(buf)
-    vim.bo[buf].bufhidden = "hide"
     if default_ft and vim.bo[buf].filetype == "" then
         vim.bo[buf].filetype = default_ft
     end