Clarify comment about variables in git_backend.pp

A couple of varibles that are not used by the git_backend.pp
manifest are defined because they're required by jeepyb.
Rewrote inline comment to make this more clear.

Change-Id: Ic96df55c40847bafcf5ce6197f1e627d7aef26aa
Merge "Clarify comment about variables in git_backend.pp"