feat: Recursive git-activity

I created git-activity in a631c98 so I can quickly answer "what have I been working on?" It walks a directory tree, finds the Git repos underneath it, and shows the latest log entries from each one in a single pass. It also supports a few filters, which is handy when I only want to review a slice of recent work; for example, from my projects folder I can run:

git-activity -r blog -m feat -x chore

That recursively scans repos, focuses on feature work, and leaves chores out of the list.