If we use a different conflict style `git rerere forget` is not able
to find the matching conflict SHA-1 because the diff generated is
actually different from what `git merge` generated, due to the
XDL_MERGE_* option differences among the codepaths.
The fix is to call git_xmerge_config() so that git_xmerge_style is set
properly and the diffs match.
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>