dirstate-v2: Reuse existing nodes when appending to a data file When writing a dirstate in v2 format by appending to an existing data file, nodes that are still "unparsed" from the previous on-disk representation have been unchanged and can therefore be reused. This makes the new data point to previously-existing data for tree nodes. Differential Revision: https://phab.mercurial-scm.org/D11095