Find commits when parent is before import merge

Switch to --simplify-merges to limit finding of commits to those that
contribute changes to the current tree which will ignore those discarded
by the previous import merge.

Use of --ancestry-path would miss commits that had a parent that was not
of the direct line between the commits being examined. This would mean
any commit created from before the merge to import the latest upstream
plus carried changes into the target branch would not be picked up on
the next import.

Remove the KNOWN-ISSUES markdown file as this was the only major known
issue that was contained within.

Change-Id: Id64e3b362b58263ec689fb5b9d0627faebcc1725
Closes-Bug: 1380520
Merge "Find commits when parent is before import merge"