mergestate: use _stateextras instead of merge records for commit related info There is a set of information related to a merge which is needed on commit. We want to store such information in the mergestate so that we can read it while committing. For this purpose, we are using merge records and introduced a merge entry state for that. However this won't scale and is not clean way to implement this. This patch reworks the existing logic related to this to use _stateextras and read from it. Right now the information stored is not very descriptive but it will be in next patch. Using _stateextras also makes MERGE_RECORD_MERGED_OTHER useless and only to be kept for BC. Differential Revision: https://phab.mercurial-scm.org/D8920