t1410-reflog.sh: avoid suppressing git's exit code in pipelines

t1410-reflog.sh: avoid suppressing git's exit code in pipelines

Piping git commands directly to wc -l suppresses the exit code of
git, hiding potential failures from the test suite. Use
test_stdout_line_count instead, which handles exit code preservation
internally while keeping the test logic clean and readable.

Signed-off-by: Gatla Vishweshwar Reddy <gatlavishweshwarreddy26@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>