t7700: drop redirections to /dev/null

t7700: drop redirections to /dev/null

Since output is silenced when running without `-v` and debugging output
is useful with `-v`, remove redirections to /dev/null as it is not
useful.

In one case where the output of stdout is consumed, redirect the output
of test_commit to stderr.

Signed-off-by: Denton Liu <redacted>
Signed-off-by: Junio C Hamano <redacted>