When building a recursive merge base, allow conflicts to occur.
Use the file (with conflict markers) as the common ancestor.
The user has already seen and dealt with this conflict by virtue
of having a criss-cross merge. If they resolved this conflict
identically in both branches, then there will be no conflict in the
result. This is the best case scenario.
If they did not resolve the conflict identically in the two branches,
then we will generate a new conflict. If the user is simply using
standard conflict output then the results will be fairly sensible.
But if the user is using a mergetool or using diff3 output, then the
common ancestor will be a conflict file (itself with diff3 output,
haha!). This is quite terrible, but it matches git's behavior.
Provide a new merge option, GIT_MERGE_TREE_FAIL_ON_CONFLICT, which
will stop on the first conflict and fail the merge operation with
GIT_EMERGECONFLICT.
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.
