rebase: add dryrun property to rebaseruntime
I want to be able to check the property in
`rebaseruntime._rebasenode()`. Passing the value via the runtime is a
convenient way.
Differential Revision: https://phab.mercurial-scm.org/D9074
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed