Use merge base for lintdiff comparison instead of main tip

This ensures lintdiff only picks up files changed on the current
branch, not files that changed on main since the branch point.

Task: 001.md

Co-authored-by: Isaac

Author: denik

tools/lintdiff.py

@@ -5,7 +5,7 @@
 """
 Drop in replacement for golangci-lint that runs it only on changed packages.
 
-Changes are calculated as diff against main by default, use --ref or -H/--head to change this.
+Changes are calculated as diff against the merge base with main by default, use --ref or -H/--head to change this.
 """
 
 import os
@@ -42,10 +42,13 @@ def main():
     if gitroot:
         os.chdir(gitroot[0])
 
-    # Get list of changed files relative to repo root.
-    # Note: Paths are always relative to repo root, even when running from subdirectories.
-    # Example: Running from tools/ returns 'tools/lintdiff.py' rather than just 'lintdiff.py'.
-    changed = parse_lines(["git", "diff", "--name-only", args.ref, "--", "."])
+    # Resolve the merge base between the ref and HEAD so that we only lint
+    # files changed on this branch, not files that changed on the ref since
+    # the branch point.
+    merge_base = parse_lines(["git", "merge-base", args.ref, "HEAD"])
+    base = merge_base[0] if merge_base else args.ref
+
+    changed = parse_lines(["git", "diff", "--name-only", base, "--", "."])
 
     cmd = args.args[:]