Handle no-newline diff markers

src/fmt.zig

@@ -6,6 +6,10 @@ const color = @import("color.zig");
 
 const Writer = std.Io.Writer;
 
+fn writeNoNewlineMarker(w: *Writer) !void {
+    try w.writeAll("\n\\ No newline at end of file\n");
+}
+
 // --- Compact diff callback ---
 
 pub const CompactCtx = struct {
@@ -48,6 +52,7 @@ pub fn compactCallback(
                 }
             }
         },
+        '<', '>', '=' => writeNoNewlineMarker(w) catch return -1,
         else => {},
     }
     return 0;
@@ -139,6 +144,7 @@ pub fn humanCallback(
                 w.writeAll(slice) catch return -1;
             }
         },
+        '<', '>', '=' => writeNoNewlineMarker(w) catch return -1,
         else => {},
     }
     return 0;

tests/conformance.sh

@@ -285,13 +285,18 @@ check "deleted file diff lines" \
 cp main_backup.py main.py
 rm main_backup.py
 
-# No trailing newline - known issue: nit's buffered writer doesn't
-# insert a newline when the file lacks one, causing lines to run together.
-# TODO: handle GIT_DIFF_LINE_DEL_EOFNL / ADD_EOFNL markers
+# No trailing newline: compact diff should keep git's marker on its own
+# line instead of running the deletion/addition lines together.
+git stash -q
 printf "no newline" > nonewline.txt
 git add nonewline.txt
+git commit -q -m "Add no-newline fixture"
 printf "no newline changed" > nonewline.txt
-git reset -q nonewline.txt
+check "no trailing newline markers" \
+  "git diff -U1 | grep -E '^[-+]no newline|^\\\\ No newline'" \
+  "$NIT diff | grep -E '^[-+]no newline|^\\\\ No newline'"
+git reset --hard -q HEAD~1
+git stash pop -q 2>/dev/null || true
 rm -f nonewline.txt
 
 # --- Show edge cases ---