Bug: handling cases where the head of PR is removed from the github

[AlbertSGS]

To start with, I just wanted to say thanks for the tool as it helped with my research a bunch.
While I was using the tool, I find the following issue:

Issue Description:

When a GitHub repository is forked and the branch is removed after sending a pull request, the tool malfunctions and does not generate any indication of the issue. The problem was detected while analyzing the pull request triggered by the following code section: openhab/openhab-addons#5664.

Proposed Solution:

To fix this issue, it is recommended to use the commit from the base repository.