merge: introduce new format for the state file This new format will allow us to address common bugs while doing special merge (graft, backout, rebase…) and record user choice during conflict resolution. The format is open so we can add more record for future usage. This file still store hexified version of node to help human willing to debug it by hand. The overhead or oversize are not expected be an issue. The old format is still used. It will be written to disk along side the newer format. And at parse time we detect if the data from old version of the mergestate are different from the one in the new version file. If its the same, both have most likely be written at the same time and you can trust the extra data from the new file. If it differs, the old file have been written by an older version of mercurial that did not knew about the new file. In that case we use the content of the old file.