Fix for JENKINS-13634

Fix for JENKINS-13634:
https://issues.jenkins-ci.org/browse/JENKINS-13634

The Git plugin would force each permutation in a matrix build to use the same SCM commit number or SHA hash (for git).

In the case of a multiple SCM Git build using a matrix configuration, this would have the unintended effect of causing all the different SCMs to use the same commit SHA and hence cause JENKINS-13634.

The fix for this problem is to remove the code forcing the same commit/SHA for each matrix configuration.

Note: This change may have some side effects in that continuous integration matrix builds might possibly use different SHAs for different matrix permutations if a commit is made while building. For release builds this is probably not so much an issue since we would generally build from a fixed tag for a release. I think this side effect is the lesser of two evils so accepting this as a slight downside/tradeoff to the fix. There may be a better way to fix this issue in the long term though...