diff: do not add excessive number of context lines to last unified-di…

…ff hunk

The last hunk could be truncated instead, but the .peekable() version is easier
to follow. If we truncated lines, we would have to adjust line ranges
accordingly.

cli/src/diff_util.rs

@@ -771,15 +771,20 @@ fn unified_diff_hunks<'content>(
         lines: vec![],
     };
     let diff = Diff::for_tokenizer(&[left_content, right_content], diff::find_line_ranges);
-    for hunk in diff.hunks() {
+    let mut diff_hunks = diff.hunks().peekable();
+    while let Some(hunk) = diff_hunks.next() {
         match hunk {
             DiffHunk::Matching(content) => {
                 let mut lines = content.split_inclusive(|b| *b == b'\n').fuse();
                 if !current_hunk.lines.is_empty() {
                     // The previous hunk line should be either removed/added.
                     current_hunk.extend_context_lines(lines.by_ref().take(num_context_lines));
                 }
-                let before_lines = lines.by_ref().rev().take(num_context_lines).collect_vec();
+                let before_lines = if diff_hunks.peek().is_some() {
+                    lines.by_ref().rev().take(num_context_lines).collect()
+                } else {
+                    vec![] // No more hunks
+                };
                 let num_skip_lines = lines.count();
                 if num_skip_lines > 0 {
                     let left_start = current_hunk.left_line_range.end + num_skip_lines;
@@ -804,12 +809,7 @@ fn unified_diff_hunks<'content>(
             }
         }
     }
-    // The last unified hunk might contain redundant "before" context lines.
-    if !current_hunk
-        .lines
-        .iter()
-        .all(|(diff_type, _line)| *diff_type == DiffLineType::Context)
-    {
+    if !current_hunk.lines.is_empty() {
         hunks.push(current_hunk);
     }
     hunks

cli/tests/test_diff_command.rs

@@ -952,14 +952,13 @@ fn test_diff_leading_trailing_context() {
 
     // N=5 <= 2 * num_context_lines: The last hunk wouldn't be split if
     // trailing diff existed.
-    // FIXME: trailing context lines should be trimmed
     let stdout = test_env.jj_cmd_success(&repo_path, &["diff", "--git", "--context=3"]);
     insta::assert_snapshot!(stdout, @r###"
     diff --git a/file1 b/file1
     index 1bf57dee4a...69b3e1865c 100644
     --- a/file1
     +++ b/file1
-    @@ -3,10 +3,10 @@
+    @@ -3,8 +3,8 @@
      3
      4
      5
@@ -969,8 +968,6 @@ fn test_diff_leading_trailing_context() {
      7
      8
      9
-     10
-     11
     "###);
 
     // N=5 > 2 * num_context_lines: The last hunk should be split no matter