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.
Is there an error message when trying to diff non-existing files?
found: No such file or directory
not: No such file or directory
Is there an error message when trying to add non-existing files?
found: No such file or directory
not: No such file or directory