[PATCH v2] t3200: replace hardcoded null OID with $ZERO_OID

To support the SHA-256 transition, replace the hardcoded 40-zero string in 'git branch --merged' with '$ZERO_OID'. The current 40-character string causes the test to fail prematurely in SHA-256 environments because Git identifies a "malformed object name" (due to the 40 vs 64 character mismatch) before it even validates the object type.

By using '$ZERO_OID', we ensure the hash length is always correct for the active algorithm. Additionally, use 'test_grep' to verify the "must point to a commit" error message, ensuring the test validates the object type logic rather than just string syntax.

Suggested-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Siddharth Shrimali <r.siddharth.shrimali@gmail.com>
---
Changes in V2:
- Updated the test to capture stderr and use 'test_grep' to verify the 
  error message. This ensures the failure is due to the object type 
  check ("must point to a commit") rather than a hash length mismatch.
- Improved the commit message to add detail to the 40 vs 64 character
  mismatch and the "premature failure" in SHA-256.

 t/t3200-branch.sh | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)