When both sides of a merge moved a submodule's gitlink, git reports it as "UU".
Pressing space used to fall into the submodule no-op guard and pop the confusing
"Nothing to stage..." error, and enter just entered the submodule, which does
nothing to resolve the superproject conflict.
Treat a conflicted submodule like the other non-textual conflicts: both space
and enter now open a picker offering the two candidate commits, "current" and
"incoming", each labelled with its summary. `git checkout --ours/--theirs` is a
no-op on gitlinks, so we resolve by checking the submodule out at the chosen
commit and staging it.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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