branchcache: don't verify all nodes while writing
nodes are verified either when they are added or used. In case of commits. we
will load the whole branchmap, only verify nodes for the branch on which we are
committing and then we write.
However before this patch, writing the branchmap was validating all the nodes
whereas it should not. This patch fixes that.
Differential Revision: https://phab.mercurial-scm.org/D6290