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>

compat/mingw.c

@@ -1086,6 +1086,12 @@ int mingw_kill(pid_t pid, int sig)
 		errno = err_win_to_posix(GetLastError());
 		CloseHandle(h);
 		return -1;
+	} else if (pid > 0 && sig == 0) {
+		HANDLE h = OpenProcess(PROCESS_QUERY_INFORMATION, FALSE, pid);
+		if (h) {
+			CloseHandle(h);
+			return 0;
+		}
 	}
 
 	errno = EINVAL;