CommitFileTreeViewModel embedded the low-level tree's SetTree, which
rebuilds the node list without touching the cursor. So after a shrinking
rebuild (e.g. moving a patch out into the index removes a file), the
selection index could be left past the end of the tree. GetSelectedItems
then indexes out of range and returns a nil node, which segfaults callers
such as canEditFiles when the options map is rendered during layout.
Override SetTree to ClampSelection after the rebuild. Unlike
FileTreeViewModel we deliberately don't also re-find the selected node by
path: that walk lands on the containing directory when a file is removed
from a dir that then collapses, whereas keeping the clamped index lands
on the sibling file (see discard_old_file_changes).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
