resolve-undo: allow plumbing to clear the information

resolve-undo: allow plumbing to clear the information

At the Porcelain level, operations such as merge that populate an
initially cleanly merged index with conflicted entries clear the
resolve-undo information upfront.  Give scripted Porcelains a way
to do the same, by implementing "update-index --clear-resolve-info".

With this, a scripted Porcelain may "update-index --clear-resolve-info"
first and repeatedly run "update-index --cacheinfo" to stuff unmerged
entries to the index, to be resolved by the user with "git add" and
stuff.

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