Merge branch 'jh/rlimit-nofile-fallback'

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