Add Reference Commits Earlier Than Reflog Remembers as a Git TIL

git/reference-commits-earlier-than-reflog-remembers.md

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+# Reference Commits Earlier Than Reflog Remembers
+
+While preparing some stats for a recent blog post on [A Decade of
+TILs](https://www.visualmode.dev/a-decade-of-tils), I ran into an issue
+referencing chuncks of time further back than 2020.
+
+```bash
+❯ git diff --diff-filter=A --name-only HEAD@{2016-02-06}..HEAD@{2017-02-06} -- "*.md"
+warning: log for 'HEAD' only goes back to Sun, 20 Dec 2020 00:26:27 -0600
+warning: log for 'HEAD' only goes back to Sun, 20 Dec 2020 00:26:27 -0600
+```
+
+This is because `HEAD@...` is a reference to the `reflog`. The `reflog` is a
+local-only log of objects and activity in the repository. That date looks
+suspiciously like the time that I got this specific machine and cloned the
+repo.
+
+In order to access this information, I need a different approach of finding
+references that bound these points in time.
+
+How about asking `rev-list` for the first commit it can find before the given
+dates in 2017 and 2016 and then using those.
+
+```bash
+❯ git rev-list -1 --before="2017-02-07 00:00" HEAD
+17db6bc4468616786a8f597a10d252c24183d82e
+
+❯ git rev-list -1 --before="2016-02-07 00:00" HEAD
+f1d3d1f796007662ff448d6ba0e3bbf38a2b858d
+
+❯ git diff --diff-filter=A --name-only f1d3d1f796007662ff448d6ba0e3bbf38a2b858d..17db6bc4468616786a8f597a10d252c24183d82e -- "*.md"
+
+# git outputs a bunch of files ...
+```