Fix panic when collaborating with new multibuffers (#29245)

Before this change, when syncing a multibuffer (such as
find-all-references) to a remote, we would renumber the excerpts from 1.
This did not matter in the past because the buffers' list of excerpts
could not change. In #27876, I added the ability for excerpts to merge,
which meant that the excerpt list could change. This manifested as
people seeing "invalid excerpt id" panics when syncing.

The initial fix to this (to re-use the excerpt ids from the host) ran
into problems because `insert_excerpts_with_ids_after` assumes that you
call it in excerpt-id order. This change de-optimizes that code to
insert the excerpts 1-by-1 in excerpt-id order, but with the
insert_after set to preserve the correct UI order.

I hope to soon remove this code path and use something more like
set-excerpts-for-path for syncing, but in the meantime we should not
panic.

Release Notes:

- Fix a panic when joining a project with a multibuffer with merged
excerpts

crates/editor/src/editor_tests.rs

@@ -12629,19 +12629,22 @@ async fn test_following_with_multiple_excerpts(cx: &mut TestAppContext) {
     // Insert some excerpts.
     leader.update(cx, |leader, cx| {
         leader.buffer.update(cx, |multibuffer, cx| {
-            let excerpt_ids = multibuffer.push_excerpts(
+            multibuffer.set_excerpts_for_path(
+                PathKey::namespaced(1, Arc::from(Path::new("b.txt"))),
                 buffer_1.clone(),
-                [
-                    ExcerptRange::new(1..6),
-                    ExcerptRange::new(12..15),
-                    ExcerptRange::new(0..3),
+                vec![
+                    Point::row_range(0..3),
+                    Point::row_range(1..6),
+                    Point::row_range(12..15),
                 ],
+                0,
                 cx,
             );
-            multibuffer.insert_excerpts_after(
-                excerpt_ids[0],
+            multibuffer.set_excerpts_for_path(
+                PathKey::namespaced(1, Arc::from(Path::new("a.txt"))),
                 buffer_2.clone(),
-                [ExcerptRange::new(8..12), ExcerptRange::new(0..6)],
+                vec![Point::row_range(0..6), Point::row_range(8..12)],
+                0,
                 cx,
             );
         });