diff: the pane reads the diff again after a rebase that finished - #98374

Summary

TLDR: after a rebase that finished, the diff pane shows the diff again instead of "Diff unavailable"

A file git leaves behind a finished rebase no longer reads as a rebase under way.

Notes

• git keeps REBASE_HEAD after a rebase that stopped on a conflict in its last commit and was then continued or skipped (seen on git 2.43 and 2.50). The pane read that file alone as a rebase under way, so every read failed and the pane said "Diff unavailable" until the file went, which could be hours.
• git's manual defines REBASE_HEAD as the commit "at which the operation is currently stopped" during a rebase (gitrevisions). By what git does when run: with that file alone git status reports nothing to commit, and it reports a rebase while the folder rebase-merge or rebase-apply is there.
• A rebase now counts as under way while REBASE_HEAD is there beside one of a rebase's folders: rebase-merge, or rebase-apply holding rebasing, which is how that folder differs from the one a stopped git am keeps.
• This only narrows what counts. Whatever showed a diff before still shows it: a stopped git am, a rebase paused at a break or at a failed exec. A merge, a cherry-pick and a revert under way read as before; none of them was seen to leave its file behind.
• A symbolic link by any of those names counts as nothing, as before.
• The pane picks the diff up at its next read: the next edit or command.

Test Plan

• claude plugin test mods/diff passes
• Rebase a branch so that its last commit conflicts, resolve, git rebase --continue; start a session there, change a file, /diff: the pane shows the diff
• Stop a rebase on a conflict and open /diff: the pane reads "Diff unavailable"
• Stop git am on a conflict and open /diff: the pane shows the diff, as before

Revert-proof: 3 new tests fail with the probe's change reverted (the mod's own suite, run against the unchanged modules); 4 more pin what must not change and pass both ways

Changelog