feat(tools/git-r): git subcommand to display r/numbers for commits

Sadly, this can't quite be an alias (which would be difficult to
automatically set up anyways), since we want to check if an r/number is
part of the (upstream) canon branch.

The test script for the subcommand doubles up as a soundness check for
our pipelines ref creation.

Change-Id: I840af6556e50187c69490668bd8a18dd7dc25a86
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8844
Tested-by: BuildkiteCI
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: flokli <flokli@flokli.de>

ops/pipelines/static-pipeline.yaml

@@ -35,6 +35,11 @@ steps:
   #
   # Revision numbers are defined as the number of commits in the
   # lineage of HEAD, following only the first parent of merges.
+  #
+  # Note that git does not fetch these refs by default, instead
+  # you'll have to modify your git config using
+  # `git config --add remote.origin.fetch '+refs/r/*:refs/r/*'`.
+  # The refs are available after the next `git fetch`.
   - label: ":git:"
     branches: "refs/heads/canon"
     command: |