Marcin Borkowski (mbork) posted a handy bit of knowledge that I didn’t know about. If you’re one of the public benefactors willing to use the development versions of Emacs and report the bugs you find, you’ll want to submit the exact version you’re using so the developers have the right context. When you’re compiling from git source that means the git commit hash.
Mbork tells us that the emacs-repository-get-version function will return the commit hash and gives us a bit of Elisp that will insert the Emacs version and commit information into the current buffer. That’s just what you need when filling out a bug report.