t4129: prevent loss of exit code due to the use of pipes

t4129: prevent loss of exit code due to the use of pipes

Piping the output of git commands like git-ls-files to another
command (grep in this case) hides the exit code returned by
these commands. Prevent this by storing the output of git-ls-files
to a temporary file and then "grep-ping" from that file. Replace
grep with test_grep as the latter is more verbose when it fails.

Signed-off-by: Chandra Pratap <redacted>
Signed-off-by: Junio C Hamano <redacted>