Merge branch 'tk/untracked-cache-with-uall' into maint

Fix for a bug that makes write-tree to fail to write out a non-existent index as a tree, introduced in 2.37. source: <20220722212232.833188-1-martin.agren@gmail.com> * tk/untracked-cache-with-uall: read-cache: make `do_read_index()` always set up `istate->repo`