upgrade to geogit 0.9-SNAPSHOT
geogit-GWC integration: truncate tiles affected by remote pushes.

The org.geogit.gwc package contains a "claspath" hook that catches
UpdateRef command calls, figures out which geogit datastores and
gwc tile layers would be affected by the ref update, for each
tile layer computes a "minimal bounds" geometry, and uses gwc truncate
tasks on each affected layer using the minimal bounds geometry as
a mask in order to adjust the truncated tiles to the regions affected
by the change to the geogit repository as much as possible.