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.
%% no changes
%% no changes
%% uncompressed contents in repo
this is a test
%% uncompress our working dir copy
this is a test
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
%% uncompress our new working dir copy
this is a test