mention shallow clone & git tag/commit verification in full clone

currently the page show:

Through Git: git clone https://salsa.debian.org/reproducible-builds/diffoscope.git

The suggestion is to:

1. change shown command to shallow clone git clone --depth 1 https://salsa.debian.org/reproducible-builds/diffoscope.git to decrease downloaded size if history is not needed.
2. full-clone is useful when user want to checkout some tag / commit, in this case it's useful to provide information on how to verify signing of tags/commits, such as using command verify-tag diffoscope-version-number and the public-keys used for signing the tags.