Consider supporting git repos with no code hosting provider

[bfabio]

pcvalidate should support git repos not belonging to a particular hosting provider and with no web interface.

[bfabio]

This is relevant only for checking the existence and conformity of referenced files inside the repo itself
(eg. logo, screenshots, etc.) where right now we get the raw URL provided by the code forge and check that one.

As a reminder for the approach we could use with git over HTTP or the native git protocol:

cd /tmp
git clone --filter=blob:none --no-checkout https://example.com/repo.git
cd repo

git checkout HEAD -- path/to/image.png

And maybe add a --its-ok-to-clone-stuff-locally because the repo can be big?