This also finally adds `bashbrew context` as an explicit subcommand so that issues with this code are easier to test/debug (so we can generate the actual tarball and compare it to previous versions of it, versions generated by `git archive`, etc).
As-is, this currently generates verbatim identical checksums to 0cde8de57d/sources.sh (L90-L96) (by design). We'll wait to do any cache bust there until we implement `Dockerfile`/context filtering:
```console
$ bashbrew cat varnish:stable --format '{{ .TagEntry.GitCommit }} {{ .TagEntry.Directory }}'
0c295b528f28a98650fb2580eab6d34b30b165c4 stable/debian
$ git -C "$BASHBREW_CACHE/git" archive 0c295b528f28a98650fb2580eab6d34b30b165c4:stable/debian/ | ./tar-scrubber | sha256sum
3aef5ac859b23d65dfe5e9f2a47750e9a32852222829cfba762a870c1473fad6
$ bashbrew cat --format '{{ .ArchGitChecksum arch .TagEntry }}' varnish:stable
3aef5ac859b23d65dfe5e9f2a47750e9a32852222829cfba762a870c1473fad6
```
(Choosing `varnish:stable` there because it currently has [some 100% valid dangling symlinks](6b1c6ffedc/stable/debian/scripts) that tripped up my code beautifully 💕)
From a performance perspective (which was the original reason for looking into / implementing this), running the `meta-scripts/sources.sh` script against `--all` vs this, my local system gets ~18.5m vs ~4.5m (faster being this new pure-Go implementation).
There's some hopefully good code comments describing this better, but essentially this changes the algorithm to have the "first" tag/entry in the SharedTag group be the "owner" of all the shared tags (instead of the following tags clobbering that and them thus being "owned" by the final Windows entry).
Before (finding tags which have both Debian *and* Ubuntu in their parentage):
```console
$ comm -12 <(bashbrew children debian | sort) <(bashbrew children ubuntu | sort) | cut -d: -f1 | sort -u
$ # (empty list, but adding "--arch-filter" makes it work:)
$ comm -12 <(bashbrew children --arch-filter debian | sort) <(bashbrew children --arch-filter ubuntu | sort) | cut -d: -f1 | sort -u
clojure
maven
neo4j
```
After:
```console
$ comm -12 <(bashbrew children debian | sort) <(bashbrew children ubuntu | sort) | cut -d: -f1 | sort -u
clojure
maven
neo4j
```
- "workflow_dispatch" so we can trigger manually
- "concurrency" so we avoid over-triggering when updating PRs/branches
- multi-line string in GITHUB_OUTPUT for better output and less flakiness
This is pulled in automatically via `gnupg`, and moved from `Recommends` to `Depends` in 99474ad900, which has been part of `src:gnupg2` since 2.1.21-4 (and every supported version of both Debian _and_ Ubuntu have 2.2.x 😇).
Since Docker's image store can't represent these, we round trip them through our self-managed (or external) containerd image store, which also makes pushing more efficient.
In other words, instead of `ubuntu:22.04`, we should store it as `docker.io/library/ubuntu:22.04` (which Docker then converts appropriately on `docker load`).
This makes it a lot easier to see/debug what's going wrong:
```console
$ BASHBREW_CACHE=/tmp/foo bashbrew fetch --arch-filter ubuntu:bionic
failed fetching git repo for "ubuntu" (tags "18.04, bionic-20230126, bionic" on arch "amd64")
failed fetching "f127810992c0981574cc137b9c83937ca1a304dc:refs/remotes/temp3848952252/temp": server does not support exact SHA1 refspec
failed finding Git commit "f127810992c0981574cc137b9c83937ca1a304dc" after fetching "refs/tags/dist-bionic-amd64-20230126:refs/remotes/temp3848952252/temp": object not found
```
Tianon Gravi