RFR: 1560: Backport command fails to load census

[Erik Joelsson]

This patch attempts to fix the problem where a commit command (in our case a /backport command) fails to instantiate a CensusInstance, due to the configured jcheck "project", at the particular commit, not existing in the census in question. What happens then is that CensusInstance.create fails, which triggers an endless series of retries in the bot.

Since none of the current commit commands have a need for the "project" part of the census, I decided to create a separate CensusInstance type `LimitedCensusInstance` without the "project", and explicitly use this in the commit commands.

Tests are passing, and I could manually verify that this solves the problem we currently have with a bad /backport command in a private GitLab instance.

-------------

Commit messages:
 - SKARA-1560

Changes: https://git.openjdk.org/skara/pull/1362/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1362&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1560
  Stats: 99 lines in 6 files changed: 55 ins; 22 del; 22 mod
  Patch: https://git.openjdk.org/skara/pull/1362.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1362/head:pull/1362

PR: https://git.openjdk.org/skara/pull/1362