t6022: don't run 'git merge' upstream of a pipe

t6022: don't run 'git merge' upstream of a pipe

The primary purpose of 't6022-merge-rename.sh' is to test 'git merge',
but one of the tests runs it upstream of a pipe, hiding its exit code.
Consequently, the test could continue even if 'git merge' exited with
error.

Use an intermediate file between 'git merge' and 'test_i18ngrep' to
catch a potential failure of the former.

Signed-off-by: SZEDER Gábor <redacted>
Reviewed-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>