diff: don't crash when merged-in addition is copied

Similar to what was explained in the previous commit, the diff code
expected copy source to be in "ctx1", which is not always the case
during a merge. This has been broken since before hg 2.0.

Also similar to the previous commit, we fix the problem by fixing up
the copy dict.

mercurial/patch.py

@@ -2273,6 +2273,11 b' def diff(repo, node1=None, node2=None, m'
     modified = sorted(modifiedset)
     added = sorted(addedset)
     removed = sorted(removedset)
+    for dst, src in copy.items():
+        if src not in ctx1:
+            # Files merged in during a merge and then copied/renamed are
+            # reported as copies. We want to show them in the diff as additions.
+            del copy[dst]
 
     def difffn(opts, losedata):
         return trydiff(repo, revs, ctx1, ctx2, modified, added, removed,

tests/test-diffdir.t

@@ -68,3 +68,10 b' parent 1, the diff should be relative to'
   +++ b/a2
   @@ -0,0 +1,1 @@
   +foo
+  $ hg diff --nodates --git
+  diff --git a/a2 b/a2
+  new file mode 100644
+  --- /dev/null
+  +++ b/a2
+  @@ -0,0 +1,1 @@
+  +foo