Generate data file of Git contributors.

Using material cut from the script git-lines-of-code-statistics.sh,
introduced the new script get-git-contributors.sh, and with it
generated the data file: itk_git_contributors.dat.

This data file is intended to be used to generate a histogram plot.
Added histogram of git contributors.

Generated using the git-contributors-histogram.py script.
Added to Results - Quality Assurance.
Merge branch 'results-quality-assurance'