Clarify assumptions in tools/lintdiff.py in comment (#3067)

## Changes

Capture thread in comments:
#3021 (comment)

Author: pietern

tools/lintdiff.py

@@ -30,9 +30,13 @@ def main():
     if args.head:
         args.ref = "HEAD"
 
+    # All paths are relative to repo root, so we need to ensure we're in the right directory.
     gitroot = parse_lines(["git", "rev-parse", "--show-toplevel"])[0]
     os.chdir(gitroot)
 
+    # 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, "--", "."])
 
     # We need to pass packages to golangci-lint, not individual files.