narrow: allow merging non-conflicting change outside of the narrow spec We use the mergestate to carry information about these merge action and reprocess them at commit time to apply the necessary update. The dirstate itself is never affected and remains "pure", with content only in the narrow-spec. This file involved in such merge are therefor not listed in `hg status`. The current testing is based on a modification of the previous testing, that refused to do such merges. As a result it is a bit simple and more extensive code and testing testing will have to be introduced later. I am planning to do this extra testing, soon. In addition, this only works for flat manifest. Support for tree manifest will need more work. I am not currently planning to do this work. Differential Revision: https://phab.mercurial-scm.org/D12119