maintenance: add --quiet option

maintenance: add --quiet option

Maintenance activities are commonly used as steps in larger scripts.
Providing a '--quiet' option allows those scripts to be less noisy when
run on a terminal window. Turn this mode on by default when stderr is
not a terminal.

Pipe the option to the 'git gc' child process.

Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>