bugfix: Fix git shortlog returning no output in non-TTY environments

[travishathaway]

Introduction

Hi maintainers 👋,

This is my first contribution to this repository, so if I have not done anything correctly, please excuse me and just let me know what to do.

I found this bug while releasing conda 26.5.0. I specifically found it because I was using rever in an opencode session where it was in a non-TTY environment.

The bug fix submitted was something that my LLM (Claude Opus 4.6) suggested to me, and I have gone even further by including regression tests that we're also written by the LLM.

Please let me know if these change are acceptable and what I can do to improve them if they are not.

Thanks!

Full description of changes

git shortlog without an explicit revision reads from stdin. In non-interactive environments (CI, subprocess calls, agent runners) stdin is empty/closed, so it exits immediately with no output. This caused git_commits_per_author and git_commits_per_email to return empty dicts, silently setting every author's num_commits to 0.

Fix by appending HEAD to the args when no since boundary is given, so git shortlog always traverses the full commit history regardless of whether stdin is a terminal. The same bug exists in both git_commits_per_author and git_commits_per_email since they share the same pattern.