expand leading tilde in GH_CONFIG path to $HOME

## Environment

    export GH_CONFIG="~/.private/config/gh/config"

## Command

    % gh checkout https://github.com/user/repo/pull/34

## Expected Result

_Branch user-branchname set up to track remote branch branchname from user._

## Actual Result

> Before Change

github.com password for wilmoore (never stored):

> After Change

_Branch user-branchname set up to track remote branch branchname from user._