Merge branch 'jh/rlimit-nofile-fallback'

When we figure out how many file descriptors to allocate for keeping packfiles open, a system with non-working getrlimit() could cause us to die(), but because we make this call only to get a rough estimate of how many is available and we do not even attempt to use up all file descriptors available ourselves, it is nicer to fall back to a reasonable low value rather than dying. * jh/rlimit-nofile-fallback: get_max_fd_limit(): fall back to OPEN_MAX upon getrlimit/sysconf failure