Wrap unique index insertion _keyExists call in a WT cursor reconfigure.

diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp

index 87bda0ace9..7eef9c8528 100644

--- a/src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp

+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp

@@ -1520,9 +1520,11 @@ Status WiredTigerIndexUnique::_insert(OperationContext* opCtx,

         setKey(c, prefixKeyItem.Get());

         ret = WT_OP_CHECK(wiredTigerCursorRemove(opCtx, c));

         invariantWTOK(ret);

-

+        c->reconfigure(c, "prefix_key=true");

         // Second phase looks up for existence of key to avoid insertion of duplicate key

-        if (_keyExists(opCtx, c, keyString.getBuffer(), sizeWithoutRecordId)) {

+        auto key_exists = _keyExists(opCtx, c, keyString.getBuffer(), sizeWithoutRecordId);

+        c->reconfigure(c, "prefix_key=false");

+        if (key_exists) {

             auto key = KeyString::toBson(

                 keyString.getBuffer(), sizeWithoutRecordId, _ordering, keyString.getTypeBits());

             auto entry = _desc->getEntry();