Cannot trigger build when there are no changes

**Describe the bug**
My latest build failed and required a change to the app build specification file to fix. I edited the configuration file in the console UI and saved it, then used the webhook to trigger a new build. The build was cancelled because there was no diff detected from previous build.

**To Reproduce**
Steps to reproduce the behavior:
1. Trigger an automated build by pushing a change to git
2. Attempt to trigger a rebuild by running a webhook curl command
3. The build is cancelled because there is no diff

**Expected behavior**
Triggering a webhook build should not be cancelled due to no diff if the previous run failed. Alternately, provide a flag in the webhook to force a rebuild.

Start with the webhook-triggered build path and the no-diff cancellation check described in the reproduction, then compare its handling of a previously failed run. Done means a webhook can trigger a rebuild after a failed run without a diff, or the webhook provides a force-rebuild option.

Written by the indexing model from the issue text.