git: on external HEAD move, do not abandon old branch

[yuja]

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (src/config-schema.json)
• I have added tests to cover my changes

[martinvonz]

Does this mean that if you create a temporary detached HEAD, it will remain visible? Something like this:

$ git checkout v1.0.0 # check out a commit to test something
$ <add printf debugging>
$ git commit -m testing
$ jj log
$ git checkout main # done testing
$ jj log # Is the "testing" commit still there?

It seems inconsistent to only add detached HEAD commits and never delete them, but maybe that's the pragmatic thing to do - it's a lot easier to notice the unwanted commit and run jj abandon on it than it is to notice a missing commit and recover it.

[yuja]

Does this mean that if you create a temporary detached HEAD, it will remain visible? Something like this:

$ git checkout v1.0.0 # check out a commit to test something
$ <add printf debugging>
$ git commit -m testing
$ jj log
$ git checkout main # done testing
$ jj log # Is the "testing" commit still there?

Correct. I don't think the testing commit should be discarded in that scenario because detached (or anonymous) branch isn't hidden in jj world.

It seems inconsistent to only add detached HEAD commits and never delete them, but maybe that's the pragmatic thing to do - it's a lot easier to notice the unwanted commit and run jj abandon on it than it is to notice a missing commit and recover it.

Yes, I agree it's inconsistent, but I think it's less scary.

[martinvonz]

Does this mean that if you create a temporary detached HEAD, it will remain visible? Something like this:

$ git checkout v1.0.0 # check out a commit to test something
$ <add printf debugging>
$ git commit -m testing
$ jj log
$ git checkout main # done testing
$ jj log # Is the "testing" commit still there?

Correct. I don't think the testing commit should be discarded in that scenario because detached (or anonymous) branch isn't hidden in jj world.

My thinking behind the current behavior (before this PR) was that it used to be visible in Git and now it's no longer visible, so we should propagate that change into jj. But as I said, I see how changing the behavior is more pragmatic.

[ilyagr]

Yeah, this seems like a good idea.