t0300: don't create unused file

Test 'credential config with partial URLs' in t0300-credentials.sh contains three "git credential fill" invocations. For two of the invocations, the test asserts presence or absence of string "yep" in the standard output. For the third test it checks for an error message in standard error. Don't redirect standard output of "git credential" to file "stdout" in t0300-credentials.sh to avoid creating an unnecessary file when only standard error is checked. Signed-off-by: Andrei Rybak <rybak.a.v@gmail.com> Reviewed-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>