t6423: fix suppression of Git’s exit code in tests

t6423: fix suppression of Git’s exit code in tests

Some test in t6423 supress Git's exit code, which can cause test
failures go unnoticed. Specifically using git <subcommand> |
<other-command> masks potential failures of the Git command.

This commit ensures that Git's exit status is correctly propogated by:
- Avoiding pipes that suppress exit codes.

Signed-off-by: Ayush Chandekar <ayu.chandekar@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>