Fix error in test_format_short_issue

[msabramo]

Fix error in test_format_short_issue:

>       return (self.fs.format(issue, bold=tc['bold'],
>       default=tc['default'])
                + extra)
E       ValueError: Invalid conversion specification

gh/commands/issue/ls.py:75: ValueError

by setting format string. The default format string has a %d in it that normally gets expanded by other code, but this doesn't happen when you call format_short_issue directly.

This fixes the Travis CI build which was failing before this (see https://travis-ci.org/sigmavirus24/github-cli/jobs/40092872 for example).

Fixes: GH-15