Improve PR Warning when base branch is not found to identify common ancestor

In cases where the git cloning strategy is limited (with a limited clone depth, with just one branch branch being cloned), SQ may run into problems when trying to identify issues for a PR analysis.

When doing a PR analysis, the scanner needs to have blame information from the source branch and for the base/target branch in order to understand which new issues have been introduced.

We currently have a shallow clone detection in place to deal with the scenarios where the source branch info is limited (see SONARSCGIT-13).

We do have a mechanism in place to detect this problem with the target branch, but the warning message could potentially be improved to suggest what might be wrong (the cloning strategy):

We should add more context by providing a link to the documentation: https://docs.sonarqube.org/latest/analysis/scm-integration/