The Local Changes tab of the Git tool window showed only the changes tree; a file's diff was visible only after opening a diff tab.
It now previews the selected file's working-tree diff beside the tree when the window is docked at the bottom and below it when docked on a side. The preview follows the selection (arrow keys included), shows a placeholder for no or multiple selection, and can be turned off with View Options › Show Diff Preview (persisted; the selection is kept when toggling).
The preview reuses the diff editor, whose toolbar did not fit a side-docked window: the editor is now a size container, the whitespace and highlighting pickers move to the settings menu below 640px and the separators go below 380px, buttons no longer shrink and the differences counter truncates.
Fixes#710.
Co-authored-by: a-rookie-of-C-language <[email protected]>
