Fix crash when joining two consecutive lines (#10000)

Release notes:

- Fixed a crash when joining two consecutive lines
([#9692](https://github.com/zed-industries/zed/pull/9692)).


This crash is not caused by `vim` or `editor`'s code logic, `join_line`
logic is okay, I found that the crash is caused by a refresh of git
`diff` after every update, hhen git diff generates hunks, it will look
for the cursor to the beginning of a line, and judge that if the cursor
result column is greater than 0, that is, it is not the beginning of a
line, it will correct the row to the next line, I think before we forgot
here that I need to adjust the column to 0 at the same time, otherwise
it is easy to go out of bounds, I am not sure if I need to add more
tests for this method, I can add if I need to, but I feel that this case
is a bit extreme

---------

Co-authored-by: Thorsten Ball <mrnugget@gmail.com>

crates/editor/src/editor_tests.rs

@@ -2685,6 +2685,65 @@ fn test_join_lines_with_multi_selection(cx: &mut TestAppContext) {
     });
 }
 
+#[gpui::test]
+async fn test_join_lines_with_git_diff_base(
+    executor: BackgroundExecutor,
+    cx: &mut gpui::TestAppContext,
+) {
+    init_test(cx, |_| {});
+
+    let mut cx = EditorTestContext::new(cx).await;
+
+    let diff_base = r#"
+        Line 0
+        Line 1
+        Line 2
+        Line 3
+        "#
+    .unindent();
+
+    cx.set_state(
+        &r#"
+        ˇLine 0
+        Line 1
+        Line 2
+        Line 3
+        "#
+        .unindent(),
+    );
+
+    cx.set_diff_base(Some(&diff_base));
+    executor.run_until_parked();
+
+    // Join lines
+    cx.update_editor(|editor, cx| {
+        editor.join_lines(&JoinLines, cx);
+    });
+    executor.run_until_parked();
+
+    cx.assert_editor_state(
+        &r#"
+        Line 0ˇ Line 1
+        Line 2
+        Line 3
+        "#
+        .unindent(),
+    );
+    // Join again
+    cx.update_editor(|editor, cx| {
+        editor.join_lines(&JoinLines, cx);
+    });
+    executor.run_until_parked();
+
+    cx.assert_editor_state(
+        &r#"
+        Line 0 Line 1ˇ Line 2
+        Line 3
+        "#
+        .unindent(),
+    );
+}
+
 #[gpui::test]
 async fn test_manipulate_lines_with_single_selection(cx: &mut TestAppContext) {
     init_test(cx, |_| {});

crates/git/src/diff.rs

@@ -130,6 +130,7 @@ impl BufferDiff {
 
             if end_point.column > 0 {
                 end_point.row += 1;
+                end_point.column = 0;
             }
 
             Some(DiffHunk {