topics: update current topic to the topic of newly rebased commit (issue5551)
The rebase code passes branchmerge equals to True while updating to the rebased
commit. We need to make sure topic is preserved even after rebase and hence we
need to update the topic even when branchmerge argument is set to True. But
there is a twist in the tale, merge also uses this part of code and we allow to
update topic when brancmerge is True, in merge cases the topic after merge will
the topic of the destination commit, not the topic of working directory parent.
So we need the function to have information about whether a rebase is going on,
and we do it by wrapping the rebase command and storing some value in the
config. This is a bit hacky but works for now. This patch fixes issue related to
loosing of topic while rebase.
Thanks to Boris Feld for the rigourous tests.