acp: Fix panic with edit file tool (#36732)

We had a frequent panic when the agent was using our edit file tool. The
root cause was that we were constructing a `BufferDiff` with
`BufferDiff::new`, then calling `set_base_text`, but not waiting for
that asynchronous operation to finish. This means there was a window of
time where the diff's base text was set to the initial value of
`""`--that's not a problem in itself, but it was possible for us to call
`PendingDiff::update` during that window, which calls
`BufferDiff::update_diff`, which calls
`BufferDiffSnapshot::new_with_base_buffer`, which takes two arguments
`base_text` and `base_text_snapshot` that are supposed to represent the
same text. We were getting the first of those arguments from the
`base_text` field of `PendingDiff`, which is set immediately to the
target base text without waiting for `BufferDiff::set_base_text` to run
to completion; and the second from the `BufferDiff` itself, which still
has the empty base text during that window.

As a result of that mismatch, we could end up adding `DeletedHunk` diff
transforms to the multibuffer for the diff card even though the
multibuffer's base text was empty, ultimately leading to a panic very
far away in rendering code.

I've fixed this by adding a new `BufferDiff` constructor for the case
where the buffer contents and the base text are (initially) the same,
like for the diff cards, and so we don't need an async diff calculation.
I also added a debug assertion to catch the basic issue here earlier,
when `BufferDiffSnapshot::new_with_base_buffer` is called with two base
texts that don't match.

Release Notes:

- N/A

---------

Co-authored-by: Conrad <conrad@zed.dev>

crates/buffer_diff/src/buffer_diff.rs

@@ -162,6 +162,22 @@ impl BufferDiffSnapshot {
         }
     }
 
+    fn unchanged(
+        buffer: &text::BufferSnapshot,
+        base_text: language::BufferSnapshot,
+    ) -> BufferDiffSnapshot {
+        debug_assert_eq!(buffer.text(), base_text.text());
+        BufferDiffSnapshot {
+            inner: BufferDiffInner {
+                base_text,
+                hunks: SumTree::new(buffer),
+                pending_hunks: SumTree::new(buffer),
+                base_text_exists: false,
+            },
+            secondary_diff: None,
+        }
+    }
+
     fn new_with_base_text(
         buffer: text::BufferSnapshot,
         base_text: Option<Arc<String>>,
@@ -213,7 +229,10 @@ impl BufferDiffSnapshot {
         cx: &App,
     ) -> impl Future<Output = Self> + use<> {
         let base_text_exists = base_text.is_some();
-        let base_text_pair = base_text.map(|text| (text, base_text_snapshot.as_rope().clone()));
+        let base_text_pair = base_text.map(|text| {
+            debug_assert_eq!(&*text, &base_text_snapshot.text());
+            (text, base_text_snapshot.as_rope().clone())
+        });
         cx.background_executor()
             .spawn_labeled(*CALCULATE_DIFF_TASK, async move {
                 Self {
@@ -873,6 +892,18 @@ impl BufferDiff {
         }
     }
 
+    pub fn new_unchanged(
+        buffer: &text::BufferSnapshot,
+        base_text: language::BufferSnapshot,
+    ) -> Self {
+        debug_assert_eq!(buffer.text(), base_text.text());
+        BufferDiff {
+            buffer_id: buffer.remote_id(),
+            inner: BufferDiffSnapshot::unchanged(buffer, base_text).inner,
+            secondary_diff: None,
+        }
+    }
+
     #[cfg(any(test, feature = "test-support"))]
     pub fn new_with_base_text(
         base_text: &str,