gc: return from cmd_gc(), don't call exit()

gc: return from cmd_gc(), don't call exit()

A minor code cleanup. Let's "return" from cmd_gc() instead of calling
exit(). See 338abb0f045 (builtins + test helpers: use return instead
of exit() in cmd_*, 2021-06-08) for other such cases.

While we're at it add a \n to separate the variable declaration from
the rest of the code in this block. Both of these changes make a
subsequent change smaller and easier to read.

This change isn't really needed for that subsequent change, but now
someone viewing that future behavior change won't need to wonder why
we're either still calling exit() here, or fixing it while we're at
it.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>