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