diff: buffer color-words context lines

I'm testing simple conflicts diffs locally, and we'll probably need to handle
consecutive context hunks when we add some form of unmaterialized conflicts
diffs. Let's buffer context hunks (up to 1 right now.) The new code looks
simpler.

cli/src/diff_util.rs

@@ -461,65 +461,52 @@ fn show_color_words_diff_hunks(
     options: &ColorWordsOptions,
 ) -> io::Result<()> {
     let line_diff = Diff::by_line([left, right]);
-    let mut line_diff_hunks = line_diff.hunks().peekable();
     let mut line_number = DiffLineNumber { left: 1, right: 1 };
+    // Matching entries shouldn't appear consecutively in diff of two inputs.
+    // However, if the inputs have conflicts, there may be a hunk that can be
+    // resolved, resulting [matching, resolved, matching] sequence.
+    let mut contexts = Vec::new();
+    let mut emitted = false;
 
-    // First "before" context
-    if let Some(DiffHunk::Matching(content)) =
-        line_diff_hunks.next_if(|hunk| matches!(hunk, DiffHunk::Matching(_)))
-    {
-        if line_diff_hunks.peek().is_some() {
-            line_number = show_color_words_context_lines(
-                formatter,
-                content,
-                line_number,
-                0,
-                options.context,
-            )?;
-        }
-    }
-    while let Some(hunk) = line_diff_hunks.next() {
+    for hunk in line_diff.hunks() {
         match hunk {
-            // Middle "after"/"before" context
-            DiffHunk::Matching(content) if line_diff_hunks.peek().is_some() => {
-                line_number = show_color_words_context_lines(
-                    formatter,
-                    content,
-                    line_number,
-                    options.context,
-                    options.context,
-                )?;
-            }
-            // Last "after" context
-            DiffHunk::Matching(content) => {
+            DiffHunk::Matching(content) => contexts.push(content),
+            DiffHunk::Different(contents) => {
+                let num_after = if emitted { options.context } else { 0 };
                 line_number = show_color_words_context_lines(
                     formatter,
-                    content,
+                    &contexts,
                     line_number,
+                    num_after,
                     options.context,
-                    0,
                 )?;
-            }
-            DiffHunk::Different(contents) => {
+                contexts.clear();
+                emitted = true;
                 line_number =
                     show_color_words_diff_lines(formatter, &contents, line_number, options)?;
             }
         }
     }
 
+    if emitted {
+        show_color_words_context_lines(formatter, &contexts, line_number, options.context, 0)?;
+    }
     Ok(())
 }
 
 /// Prints `num_after` lines, ellipsis, and `num_before` lines.
 fn show_color_words_context_lines(
     formatter: &mut dyn Formatter,
-    content: &[u8],
+    contents: &[&BStr],
     mut line_number: DiffLineNumber,
     num_after: usize,
     num_before: usize,
 ) -> io::Result<DiffLineNumber> {
     const SKIPPED_CONTEXT_LINE: &str = "    ...\n";
-    let mut lines = content.split_inclusive(|b| *b == b'\n').fuse();
+    let mut lines = contents
+        .iter()
+        .flat_map(|content| content.split_inclusive(|b| *b == b'\n'))
+        .fuse();
     for line in lines.by_ref().take(num_after) {
         show_color_words_line_number(formatter, Some(line_number.left), Some(line_number.right))?;
         show_color_words_inline_hunks(formatter, &[(DiffLineHunkSide::Both, line.as_ref())])?;