fix: plugin test git-ref default to the default branch of the plugin's repository

[javiergarea]

Summary

It implements the strategy followed by #800 to retrieve the plugin's default branch instead of relying on master.

Fixes: #974

[hyperupcall]

I'm not a maintainer, only a frequent contributor - thanks for the patch - I noticed an error with it:

This code changes the behavior in cases where plugin_name and plugin_url are empty. Instead of a nice error message, a weird git error will show instead. Since this code executes git and is now a point of failure (ex. if plugin_url is empty), this code should idealy be placed at least after this error check or right before plugin_gitref is used.

Another thing to note, is that there is another place in the code with similar functionality, and this sort of thing could be extracted into it's own function. That said, I'm not sure if that is necessary right now, and this kind of thing can be tracked in #1697 anyways.

[jthegedus]

Thanks all for raising and reviewing 🙏