git-squash: support Bash from Windows Git

Bash from the Windows Git distribution does not support process
substitution:

    $ git squash
    d:\ext\git-branch-tools\git-squash: cannot make pipe for process substitution: Function not implemented
    d:\ext\git-branch-tools\git-squash: cannot make pipe for process substitution: Function not implemented
    d:\ext\git-branch-tools\git-squash: line 39: <(git log --format="%s" -n $nrefs): ambiguous redirect
    Number to squash in:

Joy. So instead, let's read `git log` straight into the 'refs' array,
and iterate it to produce the menu.