Don't retain commit messages during merge base tests

We don't need the commit messages, just to walk the graph.
Discarding the bodies can save some memory in the JVM while
the task executes.

Change-Id: If2774e1011f8b11d516984f16b86b116a80cf1b0
Signed-off-by: Shawn O. Pearce <sop@google.com>
1 file changed