FR: `jj git import` local detached git objects by exact name

[thoughtpolice]

The josh-filter tool from the Josh project can rewrite the current HEAD of a Git repository into a new FILTERED_HEAD reference i.e. the file .git/FILTERED_HEAD is created and points to a new commit with its own history, which exists as its own tree independent of the existing history.

I would like to import this FILTERED_HEAD reference to play with in jj, however, to do that right now, you have to create a temporary branch, jj git import, then delete the branch. So something like:

git branch tmp-filtered FILTERED_HEAD
jj git import
jj branch forget tmp-filtered

Now I can:

jj workspace add ws/filtered -r $(cat .git/FILTERED_HEAD)

And get a workspace created for that reference to examine/test it. I can then jj abandon the commits when I'm done or what have you.

Proposed UX

I would like to avoid the fiddling with the underlying Git repository and branch structure. So I would like something like this:

jj git import -r FILTERED_HEAD

Where the syntax -r <GITREF> looks up and imports a commit, and possibly its ancestry, into jj's view. GITREF should more or less be treated as Git would interpret it, IMO. So you can refer to any commit ID or pointer to an ID; FILTERED_HEAD, a prefix, etc.

It would also be reasonable to support multiple -r options at once, the semantics resulting in an aggregate "import all of this in bulk", of course.

[yuja]

Where the syntax -r <GITREF> looks up and imports a commit, and possibly its ancestry, into jj's view. GITREF should more or less be treated as Git would interpret it, IMO. So you can refer to any commit ID or pointer to an ID; FILTERED_HEAD, a prefix, etc.

It sounds like a low-level command (like jj git push-commit discussed in #2098), and I think it's better to not overload jj git import command.

Implementation-wise, maybe we can just git_backend.import_head_commits(new_heads); mut_repo.add_heads(new_heads);. The existing git::import_refs() machinery has complex logic to track (named) branch movement, which isn't needed to import new anonymous branches.

[thoughtpolice]

This is getting pretty annoying for two of us at work suddenly, because we're demoing Gerrit, and to download patches it encourages you to do something like the following (in Gitspeak):

git fetch $URL refs/changes/$CHANGENO/$CHANGENO/$PATCHNO
git checkout FETCH_HEAD

So, obviously having to do the whole branch-sync-delete dance is really tiring and explaining the workaround is annoying. I'll probably take a stab at fixing this shortly.

What do we think about the name? Maybe I can come up with something, but I wonder if changing the name around to jj git commit pull instead is better? pull-commit seems odd. We could also have a jj git commit push and other various jj git commit commands along the lines of #2098. (Even though #2845 obsoletes that particular change, and I plan on adding a simple jj gerrit receive which would better address this, there are probably many cases for basic Git interop like the original bug where this would be nice, and it's much more straightforward to design, at least.)

[yuja]

What do we think about the name? Maybe I can come up with something, but I wonder if changing the name around to jj git commit pull instead is better? pull-commit seems odd. We could also have a jj git commit push and other various jj git commit commands along the lines of #2098.

commit sounds like a verb. I don't have better suggestion, but if we choose sub-sub command namespace style, the name should be unambiguously noun. jj git plumbing might be even better. :)

git fetch $URL refs/changes/$CHANGENO/$CHANGENO/$PATCHNO
git checkout FETCH_HEAD

fwiw, these commands should work fine in colocated repo. I often do gh pr checkout to pull commits for review.