According to this post from January, source indexing was "not supported in TFS Builds running against Git source control". Have any of the updates made this possible yet?
[disclosure - I work on the new build system]
git source indexing is available in the new 2015 build (note this is a completely new modern build system). It's in RC now and shipping soon.
More information available here: http://aka.ms/vsopreview
specifically: https://msdn.microsoft.com/Library/vs/alm/Build/vs/define-build
Note that if you create a VS definition from the template in the new build system, git source indexing step is there by default.
It will embed a command line for 'tf git view' in the pdb. That command line is available in VS 2015 tf command line.
The task script is open sourced here: https://github.com/Microsoft/vso-agent-tasks/blob/master/Tasks/PublishSymbols/PublishSymbols.ps1