When simplifying an odd merge, such as one that used "-s ours", we may
find ourselves TREESAME to apparently redundant parents. Prevent
simplify_merges() from removing every TREESAME parent; if this would
happen reinstate the first TREESAME parent - the one that the default
log would have followed.

This avoids producing a totally disjoint history from the default log
when the default log is a better explanation of the end result, and aids
visualisation of odd merges.
Signed-off-by: NKevin Bracey <kevin@bracey.fi>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>