[BUG] git help -a should no use page when in pipe

To whom may concern,

Since git 2.20.0, invoking `git help -a` will cause git using page (e.g. less) to display help message. However, such behaviour is not desirable when `git help -a` is run inside a pipe or invoked by another program.

Step to reproduce the bug:
     git help -a | cat # this freezes the command line prompt.

This bug also causes issues in `hub` when invoking `git <alias>`. More detail can be found at https://github.com/github/hub/issues/1963

I would suggest `git help -a` only invokes page when stdout is a tty.

Thanks for your attention.

Best Regards, Cheng