feat: default diffs to branch changes

docs/editing.md

@@ -313,11 +313,14 @@ Run `:diff` in Normal mode, choose **Before**, then **After**, and press Enter t
 
 The floating selector has **Revisions / Bookmarks** tabs for jj, or **Commits / Branches / Tags** for Git. Type to filter by revision/change ID, commit ID, reference name, or description. **Working checkout** stays first in every category, even when it does not match the query. As Before it is a disk snapshot, including eligible untracked files. As After it includes unsaved regular-text buffers and enables live editing; opening a comparison never saves them.
 
+New comparisons default to **Branch changes**: Before becomes the unique merge base of the selected Before and After revisions, and After stays the selected tip. Changes exclusive to the Before branch are excluded. The Before border identifies both selected names and the immutable merge-base commit ID. **Snapshots** compares the two selected snapshots directly. **Ctrl+b** toggles the mode, also available as **`:diff.mode.toggle`**, the palette action, and remappable **`config.core.diff.mode.toggle`**. Toggling an opened comparison recomputes using the original selected endpoints. Selecting **Working checkout** on either side immediately forces **Snapshots**; Branch changes is unavailable while it is selected. Missing or multiple merge bases produce an explicit, recoverable selector error rather than silently falling back; choose Snapshots to compare such histories. jj's synthetic root is the empty ancestor of every jj history.
+
 | Selector input | Effect |
 | --- | --- |
 | Tab / Shift+Tab | Next / previous category |
 | Up / Down, Ctrl+k / Ctrl+j | Previous / next entry |
 | Backspace / Ctrl+u | Remove a query grapheme / clear query |
+| Ctrl+b | Toggle Branch changes / Snapshots (Working checkout requires Snapshots) |
 | Enter | Choose the entry |
 | Esc | Close either selector stage, cancel outstanding work, and restore the source buffer without discarding edits |