feat(sterni/blërg/git): render html, org and md entries

Markup for the git backend is implemented as a lookup table to commands
that read markup on stdin and write it to stdout. Markdown uses lowdown,
Org pandoc and HTML doesn't need to be altered.

Future Work:

- Title extraction from the files instead of the file name. Since we use
  the file name as ids in the git backend, it is currently impossible to
  change the title without breaking links.
- Use emacs for Org. Unfortunately, it is really slow, so we may need to
  do something drastic, like writing an sourcegraph style markup
  conversion service or dumping an emacs image.
- Copy post dependencies, e.g. included images.

Change-Id: I88ad4cc18ada1d68d81fdecf685096f105e3fef3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13110
Tested-by: BuildkiteCI
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: sterni <sternenseemann@systemli.org>

users/sterni/blërg/README.md

@@ -5,13 +5,21 @@
 - [CBQN][] (other [BQN][] implementations may work, but are untested)
 - Marshall Lochbaum's [bqn-libs][] which blërg expects to find at the
   location the `BQN_LIBS` environment variable points to.
-- //users/sterni/mn2html
-- [mblaze(7)][mblaze]
 - [execline][]
 - POSIX `printf(1)` (e.g. from GNU coreutils)
+- `mail-notes` backend
+  - //users/sterni/mn2html
+  - [mblaze(7)][mblaze]
+- `git` backend
+  - [git][]
+  - [lowdown][] for Markdown support
+  - [pandoc][] for Org Mode support
 
 [mblaze]: https://github.com/leahneukirchen/mblaze/
 [execline]: https://skarnet.org/software/execline/
 [BQN]: https://mlochbaum.github.io/BQN/
 [CBQN]: https://github.com/dzaima/cbqn
 [bqn-libs]: https://github.com/mlochbaum/bqn-libs/
+[lowdown]: https://kristaps.bsd.lv/lowdown/
+[pandoc]: https://pandoc.org/
+[git]: https://git-scm.com/

users/sterni/blërg/blërg.bqn

@@ -42,6 +42,7 @@ LR ← Lines∘R
 GetEnv ← {R "importas"‿"env"‿𝕩‿"printf"‿"%s"‿"$env"}
 
 RelPath ← •wdpath⊸•file.At
+Ext ← '.'⊸(⊑∘1⊸↑∘/∘(=∧∊∘⊢)↑⊢)⌾⌽ •file.Name
 
 # 3p dependencies
 
@@ -79,27 +80,42 @@ MailNotesBackend ← {𝕊 config:
 
 # Git Backend
 
+converters ← ⍉>⟨
+# TODO(sterni): avoid cat
+⟨"html", ⋈"cat"⟩,
+⟨"md", "lowdown"‿"-T"‿"html"‿"--html-no-skiphtml"‿"--html-no-escapehtml"‿"--html-callout-mdn"⟩,
+# TODO(sterni): use emacs
+⟨"org", "pandoc"‿"-f"‿"org"‿"-t"‿"html5"⟩,
+⟩
+
+# TODO(sterni): don't assemble blocks in this ad hoc fashion
+# TODO(sterni): pipefail
+PipelineCmd ← {"pipeline"⋈⊸∾(' '⊸∾¨𝕨)∾""⋈⊸∾𝕩}
+
 GitBackend ← {𝕊 config:
   repo ← RelPath •file.At config j.ObjGet "repository"
   path ← ∾⟜'/' '/' StripRight config "." j._ObjGetDef "path"
 
   # We use zero separated fields when dealing with paths, so quoting is unnecessary
-  Git ← {R "git"‿"-c"‿"core.quotePath=false"‿"-C"‿repo∾𝕩}
-  rev ← Git "rev-parse"‿"HEAD"
+  GitCmd ← {"git"‿"-c"‿"core.quotePath=false"‿"-C"‿repo∾𝕩}
+  rev ← R GitCmd "rev-parse"‿"HEAD"
   # Use the author date of the latest commit on the file to establish the date
   # of the file. The author date is easier to arbitrarily change and survives
   # history rewrites. It could be interesting to ignore commits that touch
   # multiple files (especially treewide ones).
-  PathDate ← {ReadPosDec Git "log"‿"--date=unix"‿"--pretty=tformat:%ad"‿"-1"‿rev‿"--"‿𝕩}
+  PathDate ← {ReadPosDec R GitCmd "log"‿"--date=unix"‿"--pretty=tformat:%ad"‿"-1"‿rev‿"--"‿𝕩}
 
   Entries ⇐ {𝕤⋄
-    blobs ← ∘‿2⥊@ SplitChar Git "ls-tree"‿"-zr"‿"--format=%(path)%x00%(objectname)"‿rev‿path
+    blobs ← ∘‿2⥊@ SplitChar R GitCmd "ls-tree"‿"-zr"‿"--format=%(path)%x00%(objectname)"‿rev‿path
     {𝕊 p‿b:
       id ⇐ Slugify path DropPrefix p
       # TODO(sterni): extract from file if possible
       title ⇐ •file.Name p
       time ⇐ PathDate p
-      Render ⇐ {𝕤 ⋄ {"<pre>"∾𝕩∾"</pre>"} Git "cat-file"‿"blob"‿b}
+      Render ⇐ {𝕤
+        conv ← converters j.ObjGet Ext p
+        R (GitCmd "cat-file"‿"blob"‿b) PipelineCmd conv
+      }
     }˘blobs
   }
 }

users/sterni/blërg/default.nix

@@ -5,8 +5,10 @@ let
   runtimeDependencies = [
     depot.users.sterni.mn2html
     pkgs.mblaze
-    pkgs.execline # execline-cd, importas
+    pkgs.execline # execline-cd, importas, pipeline
     # coreutils   # for printf (assumed to be installed)
+    pkgs.pandoc
+    pkgs.lowdown
   ];
 
   # … and this