Clean up diff patch temporary file

[luojiyin1987]

Part of #34067.

Summary

Register an EXIT trap for the RCS diff temporary file.
Remove the normal-path cleanup at the end.
The trap now handles every exit path.

Root cause

The script only removed the file after all commands succeeded.
The set -e option can stop the script before that cleanup.

Impact

Early failures and interruptions no longer leave the diff file behind.
Patch generation behavior stays unchanged.

Checks

• bash -n tools/make-diffpatch.sh
• git diff --check
• A forced Git failure leaves the file with the original script.
• The same failure removes the file with the updated script.