[JENKINS-66480] Add fallback reference build for multibranch projects.

[uhafner]

If the reference build detection does not work because the number of recorded commits is not large enough, then as a
fallback for pull requests the last build of the target branch will be returned as reference build.

[codecov]

Codecov Report

Merging #368 (6b54f54) into master (36f41ba) will increase coverage by 0.63%.
The diff coverage is 89.09%.

@@             Coverage Diff              @@
##             master     #368      +/-   ##
============================================
+ Coverage     80.12%   80.76%   +0.63%     
- Complexity      157      162       +5     
============================================
  Files            22       22              
  Lines           644      681      +37     
  Branches         69       73       +4     
============================================
+ Hits            516      550      +34     
- Misses          100      104       +4     
+ Partials         28       27       -1     

Impacted Files	Coverage Δ
.../forensics/git/reference/GitReferenceRecorder.java	89.58% <82.75%> (+16.25%)	⬆️
...gins/forensics/git/reference/GitCommitsRecord.java	83.54% <96.15%> (+5.21%)	⬆️
...gins/forensics/git/miner/CommitStatisticsStep.java	67.10% <0.00%> (-10.53%)	⬇️
...plugins/forensics/git/miner/MergeBaseSelector.java	86.66% <0.00%> (+13.33%)	⬆️

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