evolve: use phases.new-commit config to retain phase information (issue5720)
Since the previous patches replaced the use of graftstate with evolvestate and
we don't graft anymore on evolve --continue, we have now the capability to fix
underlying bugs. This patch makes sure we use the phases.new-commit config to
make a new commit of the same phase as that of the predecessor.
Tests added in the previous patch shows the bug being fixed.