Add missing RevWalk.reset() after checking merge base

This fixes an exception from RevWalk when trying to push a new
commit for review.  RevWalk won't allow changing the RevFilter back
to the default of RevFilter.ALL while the walk still appears to be
running. A reset() first fixes that.

Change-Id: Ic12022569fafbb0693199e9663e66b0e07e451b0
1 file changed