builtin/gc: avoid global state in `gc_before_repack()`

builtin/gc: avoid global state in `gc_before_repack()`

The `gc_before_repack()` should only ever run once in git-gc(1), but we
may end up calling it twice when the "--detach" flag is passed. The
duplicated call is avoided though via a static flag in this function.

This pattern is somewhat unintuitive though. Refactor it to drop the
static flag and instead guard the second call of `gc_before_repack()`
via `opts.detach`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>