Allow specifying a different style for diff indicator in vcs gutter. (#8343)

This allows using a background in diff style
(for nice patch file coloring) while keeping the
gutter indicator nice (and using appropriate colors).

book/src/themes.md

@@ -245,9 +245,12 @@ We use a similar set of scopes as
 
 - `diff` - version control changes
   - `plus` - additions
+    - `gutter` - gutter indicator
   - `minus` - deletions
+    - `gutter` - gutter indicator
   - `delta` - modifications
     - `moved` - renamed or moved files/changes
+    - `gutter` - gutter indicator
 
 #### Interface
 

helix-view/src/gutter.rs

@@ -94,9 +94,9 @@ pub fn diff<'doc>(
     theme: &Theme,
     _is_focused: bool,
 ) -> GutterFn<'doc> {
-    let added = theme.get("diff.plus");
-    let deleted = theme.get("diff.minus");
-    let modified = theme.get("diff.delta");
+    let added = theme.get("diff.plus.gutter");
+    let deleted = theme.get("diff.minus.gutter");
+    let modified = theme.get("diff.delta.gutter");
     if let Some(diff_handle) = doc.diff_handle() {
         let hunks = diff_handle.load();
         let mut hunk_i = 0;