Discarding lines from a commit's diff rewrites the commit with a
rebase. For this we look up the commit among the commits of the
current branch. If it isn't there, pressing the key does nothing. The
disabled reason only allows discarding in a diff of the commits panel,
so this shouldn't happen. If it does, the user should see it.
Show an error that names the commit instead.
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>