Speaking personally, I've always used the Windows GUI git tools, but command line principles should be the same.

If you've cloned BOINC, you already have everything - master, all branches, all tags. From there, I export the code for tag 7.8.3 (or whatever), which gives me a single .zip file containing the code as at that version only. Then, I expand the zip into a new, separate, folder tree from where I can build, test modifications or anything else. Getting fixes back into the github source is another question entirely.

Edit - should have said. v7.8.3 is a bit of a dog's breakfast, and is missing a lot of bugfixes which should have been included - you'll find a pull request from me (#2065) attempting to identify the fixes which were missed. Conversely, master (and hence 7.9.0) contains a lot of new development code which has never been tested: it doesn't surprise me it's broken. Best of luck.