evolve: extract logic to new method _evolvemerge
This patch introduces a new method _evolvemerge to merge orig to dest in
relocate method.
This simplifies the code of the method relocate and allows us to re-use it
later in the scope of the rework of 'hg evolve --continue'
recursive-include docs/figures *.svg
include docs/figures/hgview-example.png
include docs/*.rst
include docs/*.py
include docs/tutorials/*.t
include docs/makefile
include docs/static/*.svg
include hgext/__init__.py
include hgext/evolve.py
include hgext/pushexperiment.py
include hgext/simple4server.py
include setup.py
include README
include COPYING
include tests/*.t
include tests/*.py
include tests/_exc-util.sh
include tests/dummyssh
exclude tests/test-oldconvert.t
exclude tests/test-qsync.t
exclude tests/test-drop.t