script/redmine-upkeep: add transform to resolve merged issue

script/redmine-upkeep: add transform to resolve merged issue

Few things:

- Add priority to transforms. Largely this is to have the "merged"
  transformation run first to update the "Merge Commit" field of the ticket
  before any other transform intends to look at that field. This avoids
  duplicating logic to set the Merge Commit field.

- Fix a bug where the github API cannot be trusted to indicate the Merge Commit
  for a PR. When the branch is rename or changed, the github backend clearly
  gets confused and gives the "HEAD" commit instead.

- Add new transform to resolve tickets that are merged to either Resolved or
  Pending Backport status.

* Note: filters on TAGS cannot be combined. There is some restructuring to deal with that.

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>