in phase alwasy uses /merge in path which always fails if config is to check on source changes

[sstelmasik]

I found the issue in the pull-request plugin for concourse...

there is a if clause for checking ref-names in the check phase - but it's fixed value in the input phase:

check: https://github.com/mmb/concourse-bitbucket-pullrequest-resource/blob/master/assets/check#L56

prq_branch="from"
if [ "$rebuild_when_target_changed" == "true" ]; then
  prq_branch="merge"
fi

in: https://github.com/mmb/concourse-bitbucket-pullrequest-resource/blob/master/assets/in#L68

branch="pull-requests/${prq_id}/merge

and listing remoles via console shows:

f6a848ff222a9a13b3113afb56a35a382f682c41        refs/pull-requests/63/from
66b38ab5659f99813103921b920125e9976886ed        refs/pull-requests/64/from

=> Result is always a failure!

fatal: Couldn't find remote ref refs/pull-requests/64/**merge**