merge: fix spurious merges for copies in linear updates
We make better use of contexts to accurately identify copies that
don't need merges.
Add a simple test and update other tests.
adding dir/bar
adding foo
dir/bar
foo
adding dir/bar_2
adding foo_2
dir/bar_2
foo_2
adding a
adding c
adding b
adding d
removing a
removing c
recording removal of a as rename to b (100% similar)