Strip excess `'`s from default prompt message

In the `prompt` function in `lib/git-hub`, the default prompt message is
specified as the default value of a `:-`-form Bash parameter expansion.

This default value was wrapped in single quotation marks (instances of
U+0027 APOSTROPHE — `'`), which were taken literally by Bash, resulting
in the prompt message displayed to the user being wrapped in these same
quotation marks — e.g.:

> Press <ENTER> to continue, or <CTL>-C to exit.