[FIXED JENKINS-17775] Find (model) parent POMs properly during fingerprint recording at end of build.

TransferResource.getRepositoryUrl is documented to end in a slash if known, so we should not add one.
Parsing POMs
Downloaded artifact http://repo.jenkins-ci.org/public//org/jenkins-ci/plugins/plugin/1.456/plugin-1.456.pom
Downloaded artifact http://repo.jenkins-ci.org/public//org/jenkins-ci/jenkins/1.26/jenkins-1.26.pom

[JENKINS-17775] If MavenProject.getParent throws ISE during fingerprinting, report it but at least proceed.
Proper fix is to ensure that the model resolution uses the same environment as the actual build,
or is otherwise more lenient about finding the parent (e.g. enables plugin resolution).

[JENKINS-16845] Refinement to hotfix: if build field has already been cleared due to earlier warning, do not throw a new NPE.

This bug indicates that people are having trouble understanding who's
responsible for aborting the build. This change records that
information through the logger.

A recent Stapler update caused the Jelly facet to ignore *.xml.
So need to use raw j:include (as plugins were doing), not st:include (as core did).

Most notable, cvs 1.6 → 2.8, to get the critical fix of removing the mail address resolver.

(cherry picked from commit 9858bf1e)