basou: Name the base a board's repos are compared with, count what is ahead of it, and say when it was last fetched

[takashi-matsuyama]

What

The repos section of a version 3 record names the branch each repo is compared with, counts the commits ahead of it, and says when it was last fetched (experimental, record_version: 3).

• behind_main is renamed behind, counted as before. The repos method stays 2.
• base: the branch of origin compared with, as origin/<branch>: the one refs/remotes/origin/HEAD names, or origin/main when it names none (the same resolution as 0.70.1). Null, with its meaning and no entry, when there is neither. When origin/HEAD names a branch that is not there, base is that name and behind / ahead are not measured, as behind_main was not.
• ahead: the commits HEAD has that the base does not (rev-list --count base..HEAD); 0 for a HEAD with no commit yet. Not measured where behind is not (a shallow clone, a HEAD that names no commit, a base that cannot be read).
• fetched_at: the mtime of git rev-parse --git-path FETCH_HEAD (this worktree's), in UTC, only when FETCH_HEAD holds the line git writes for the base's branch of origin's URL at the commit the base is at now (<oid>\t[not-for-merge]\tbranch '<branch>' of <url>). Otherwise null with no entry: no FETCH_HEAD, an empty one (a failed fetch), no such line (a fetch since from another URL, or of other branches only), or one at another commit (a fetch in another worktree, a push since). A base, a FETCH_HEAD or origin's URL that cannot be read is an entry, as other gaps are. A later git fetch --append moves the time and leaves origin's line, and a remote at origin's URL writes the same line as origin; the docs say both.
• The URL is compared as git writes it there: the user and password of a URL with a scheme or of user@host:path taken out, then the slashes at its end, then one .git (git's transport_anonymize_url and the trimming after it). A URL this does not reproduce gives null, never a time. This closes the case of a remote at another URL whose branch of the same name is at the same commit.
• fetched_at is left out of the digest and of the difference from the previous record: a fetch that brings nothing still rewrites FETCH_HEAD. Its not_found entries stay in both: they follow a ref or a file that cannot be read, not a fetch.

Also: basou board measure prints them on each repo's line (base origin/main, behind 1, ahead 0, fetched <time>, or no base (...)); the board page measured on the spot draws them as columns (a recorded board page draws no repos table, so records of version 1 and 2 need nothing there); basou board guide says what the four mean in step 2, and in the principles for 04 that ahead above 0 is not only commits not pushed and is read with branch; the record version comment and the CHANGELOG (Unreleased).

basou still sends nothing: git remote get-url origin and FETCH_HEAD are read locally.

Tests

• New in measure.test.ts: ahead with merges; fetched_at for a not-for-merge line; null with no entry for a clone never fetched, a failed fetch, a fetch of another branch only, a fetch of another remote, at another URL, whose main is at the same commit, and a push; the FETCH_HEAD of the worktree measured (a linked worktree's absolute path); origin URLs fetched over an ssh stand-in that runs the command locally (user@host:path/, a password, git+ssh, '@' in the path, a local path), held against what git itself wrote; hand-written lines for another URL, a tag, a scheme git does not take, several trailing slashes, a local path with '@' and ':', and a name of five characters; an unreadable FETCH_HEAD (EACCES, EISDIR); a git that cannot say where FETCH_HEAD is; the digest unmoved by a later or a failed fetch.
• Existing repos tests moved to the new fields, with entries for base, ahead and fetched_at where they now apply.
• diff.test.ts: fetched_at is not compared; against a record of version 1 or 2, behind_main is removed and base, behind, ahead added.
• live.test.ts, view.test.ts, board.test.ts, guide.test.ts: the page's columns and their classes, the CLI line, the guide's text.
• Second commit (review follow-ups): a branch and an origin path that are not ASCII; removing only one .git; the not_found entries of fetched_at staying in the digest; a git remote get-url stopped by a signal is an entry; a later --append fetch and a remote at origin's URL, as the docs now say.