Recently introduced resolve_undo_read() expected arithmetic to (void *)
to work on byte-addresses.  Correct this.

Noticed by Brandon Casey.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Once you resolved conflicts by "git add path", you cannot recreate the
conflicted state with "git checkout -m path", because you lost information
from higher stages in the index when you resolved them.

Since we record the necessary information in the resolve-undo index
extension these days, we can reproduce the unmerged state in the index and
check it out.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

When resolving a conflict using "git add" to create a stage #0 entry, or
"git rm" to remove entries at higher stages, remove_index_entry_at()
function is eventually called to remove unmerged (i.e. higher stage)
entries from the index.  Introduce a "resolve_undo_info" structure and
keep track of the removed cache entries, and save it in a new index
extension section in the index_state.

Operations like "read-tree -m", "merge", "checkout [-m] <branch>" and
"reset" are signs that recorded information in the index is no longer
necessary.  The data is removed from the index extension when operations
start; they may leave conflicted entries in the index, and later user
actions like "git add" will record their conflicted states afresh.

Signed-off-by: Junio C Hamano <gitster@pobox.com>