Merge branch 'jc/unresolve-removal'

"checkout --merge -- path" and "update-index --unresolve path" did not resurrect conflicted state that was resolved to remove path, but now they do. * jc/unresolve-removal: checkout: allow "checkout -m path" to unmerge removed paths checkout/restore: add basic tests for --merge checkout/restore: refuse unmerging paths unless checking out of the index update-index: remove stale fallback code for "--unresolve" update-index: use unmerge_index_entry() to support removal resolve-undo: allow resurrecting conflicted state that resolved to deletion update-index: do not read HEAD and MERGE_HEAD unconditionally