As discussed in the last Nix team meeting (2024-02-95), this method
doesn't belong because `CanonPath` is a virtual/ideal absolute path
format, not used in file systems beyond the native OS format for which a
"current working directory" is defined.
Progress towards #9205
Enabled for fetchGit, which historically had this behavior,
among other behaviors we do not want in fetchGit.
fetchTree disables this parameter by default. It can choose the
simpler behavior, as it is still experimental.
I am not confident that the filtering implementation is future
proof. It should reuse a source filtering wrapper, which I believe
Eelco has already written, but not merged yet.
GitArchiveInputScheme now streams tarballs into a Git repository. This
deduplicates data a lot, e.g. when you're fetching different revisions
of the Nixpkgs repo. It also warns if the tree hash returned by GitHub
doesn't match the tree hash of the imported tarball.
Instead of making a complete copy of the repo, fetching the
submodules, and writing the result to the store (which is all
superexpensive), we now fetch the submodules recursively using the Git
fetcher, and return a union accessor that "mounts" the accessors for
the submodules on top of the root accessor.
Since a26be9f3b8, the same parser is used
to parse the result of sourcehut’s `HEAD` endpoint (coming from [git
dumb protocol]) and the output of `git ls-remote`. However, they are very
slightly different (the former doesn’t specify the current reference
since it’s implied to be `HEAD`).
Unify both, and make the parser a bit more robust and understandable (by
making it more typed and adding tests for it)
[git dumb protocol]: https://git-scm.com/book/en/v2/Git-Internals-Transfer-Protocols#_the_dumb_protocol
