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.

I understand if you don't want to accept patches which dirty the code to gain Windows support. This is the best I could come up with, but maybe someone who speaks better Bash can think of a better way?