DiffAlgorithms can return different edit locations for inserts or
deletes, if they can be "shifted" up or down repeating blocks of
lines. This causes the 3-way merge to apply both edits, resulting in
incorrectly removing or duplicating lines.
Augment an existing "tidy-up" stage in DiffAlgorithm to move all
shiftable edits (not just the last INSERT edit) to a consistent
location, and add test cases for previously incorrect merges.
Bug: 514095
Change-Id: I5fe150a2fc04e1cdb012d22609d86df16dfb0b7e
Signed-off-by: KB Sriram <kbsriram@google.com>