More or less, I created this scenario on purpose:
# We use git-flow.
$ git flow feature start <feature>
# update a file
$ git commit <file> -m '<message>'
$ git rebase -i develop
$ git push -u origin <feature-branch>
# Try arcanist now
$ arc diff
# diff was accepted
$ git flow feature finish <feature>
# feature branch removed, now in develop, let's push
$ git push
$ arc land
Branch "<feature-branch>" does not exist in the local working copy.
$ git fetch -a
$ git checkout <feature-branch>
$ arc land
Landing current branch '<feature-branch>'.
TARGET Landing onto "<feature-branch>", selected by following tracking branches upstream to the closest remote.
REMOTE Using remote "origin", selected by following tracking branches upstream to the closest remote.
FETCH Fetching origin/<feature-branch>...
Usage Exception: There are no commits on "<feature-branch>" which are not already present on the target.
arc close-revision <Diff-ID> should close it. Basically you have two competing Git add-ons that are "landing" your changes.
git flow feature finish <feature> already "landed" your changes so when you run
arc land, it isn't able to perform its primary function of "landing" your changes.