copies: pass changectx instead of manifest to _computenonoverlap

The _computenonoverlap function takes two manifests to allow extensions to hook
in and read the manifest nodes produced by the function. The remotefilelog
extension actually needs the entire changectx instead (which includes the
manifest) so it can prefetch the subset of files necessary for a sparse checkout
(and the sparse checkout depends on which commit is being accessed, hence the
need for the changectx).

I have tests in the remotefilelog extension that cover this.

mercurial/copies.py

@@ -209,12 +209,12 b' def pathcopies(x, y):'
         return _backwardrenames(x, y)
     return _chain(x, y, _backwardrenames(x, a), _forwardcopies(a, y))
 
-def _computenonoverlap(repo, m1, m2, addedinm1, addedinm2):
-    """Computes, based on addedinm1 and addedinm2, the files exclusive to m1
-    and m2. This is its own function so extensions can easily wrap this call
+def _computenonoverlap(repo, c1, c2, addedinm1, addedinm2):
+    """Computes, based on addedinm1 and addedinm2, the files exclusive to c1
+    and c2. This is its own function so extensions can easily wrap this call
     to see what files mergecopies is about to process.
 
-    Even though m1 and m2 are not used in this function, they are useful in
+    Even though c1 and c2 are not used in this function, they are useful in
     other extensions for being able to read the file nodes of the changed files.
     """
     u1 = sorted(addedinm1 - addedinm2)
@@ -310,7 +310,7 b' def mergecopies(repo, c1, c2, ca):'
 
     addedinm1 = m1.filesnotin(ma)
     addedinm2 = m2.filesnotin(ma)
-    u1, u2 = _computenonoverlap(repo, m1, m2, addedinm1, addedinm2)
+    u1, u2 = _computenonoverlap(repo, c1, c2, addedinm1, addedinm2)
 
     for f in u1:
         ctx = setupctx(c1)