The default merge_file level was XDL_MERGE_MINIMAL, which will
produce conflicts where there should not be in the case where
both sides were changed identically. Change the defaults to be
more aggressive (XDL_MERGE_ZEALOUS) which will more aggressively
compress non-conflicts. This matches git.git's defaults.
Increase testing around reverting a previously reverted commit to
illustrate this problem.
