gc: reject if another gc is running, unless --force is given

gc: reject if another gc is running, unless --force is given

This may happen when `git gc --auto` is run automatically, then the
user, to avoid wait time, switches to a new terminal, keeps working
and `git gc --auto` is started again because the first gc instance has
not clean up the repository.

This patch tries to avoid multiple gc running, especially in --auto
mode. In the worst case, gc may be delayed 12 hours if a daemon reuses
the pid stored in gc.pid.

kill(pid, 0) support is added to MinGW port so it should work on
Windows too.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>