It's not always obvious the mapping between stage level and
conflict-ness. More importantly, this can lead otherwise sane
people to write constructs like `if (!git_index_entry_stage(entry))`,
which (while technically correct) is unreadable.
Provide a nice method to help avoid such messy thinking.
The "merge none" (don't automerge) flag was only to aide in
merge trivial tests. We can easily determine whether merge
trivial resulted in a trivial merge or an automerge by examining
the REUC after automerge has completed.
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.
