Enable Maven reproducible builds

- configure Maven to run build reproducibly [1]
- use UTC timestamp of checked out commit as build timestamp
- add git-describe, git-commit-id, git-commit-id, git-tags,
  git-remote-origin-url to MANIFEST.MF files
- configure cyclonedx-maven-plugin to also use UTC timestamp of
  checked out commit
- for packaging build use tycho-buildtimestamp-jgit [2] to ensure
  version uses the timestamp of the last commit
- SBOMs are not reproducible by design [3] they should have a build
  timestamp matching the time when the build was executed and a serial
  number which is a unique UUID per build run. Hence exclude them from
  comparison [4].
- Use gmavenplus-plugin to format build timestamps. Maven expects
  build timestamp in ISO-8601 format, to replace the qualifier in
  versions the timestamp format must be compatible with rules for OSGi
  version numbers. Didn't find a way to read the properties set by the
  git-commit-id-maven-plugin from another plugin. Hence use JGit in a
  groovy script to get the commit time of the current HEAD and provide
  it in these two formats.

TODO: packaging build (features and p2 repository) is not yet binary
reproducible since that's not yet supported by Tycho [5], artefacts have
reproducible version numbers but file lastModified timestamps are not
yet reproducible.

Test plan for Maven build:
- build using
  mvn clean install"
- verify second build is reproducible:
  mvn -T1 clean verify artifact:compare
  verification seems not to be thread-safe, hence run it with a single
  thread using option -T1

For packaging build (still fails due to non-reproducible file
timestamps):
- build using
  mvn -f org.eclipse.jgit.packaging/pom.xml clean install
- verify second build is reproducible:
  mvn -T1 -f org.eclipse.jgit.packaging/pom.xml clean verify artifact:compare

[1] https://maven.apache.org/guides/mini/guide-reproducible-builds.html
[2] https://wiki.eclipse.org/Tycho/Reproducible_Version_Qualifiers
[3] https://github.com/CycloneDX/cyclonedx-maven-plugin/issues/84
[4] https://maven.apache.org/plugins/maven-artifact-plugin/compare-mojo.html
[5] https://github.com/eclipse-tycho/tycho/issues/233

Change-Id: I0202f55a1b6ae0edd922cfef638beb39d2ce9417

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