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.
abort: repository a not found!
255
abort: error: Connection refused
255
abort: repository a not found!
255
abort: destination '../a' already exists
255
abort: repository a not found!
255
destination directory: q
abort: destination 'q' already exists