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 a/f
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
copying a/f to b/f
removing a/f
adding a/aa/g
pulling from ../r2
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
(run 'hg heads' to see heads, 'hg merge' to merge)
2 files updated, 0 files merged, 1 files removed, 0 files unresolved
(branch merge, don't forget to commit)
M b/f
A b/aa/g
a/aa/g
R a/aa/g
R a/f