copies: filter out copies when target is not in destination manifest

When chaining a series of commits that copied a file with a series
that removed the destination file, we would still include the copy in
the result. Similar to the previous patch, I have checked that `hg
status --copies` is not affected by this bug, but I wouldn't be
surprised if some commands are.

Differential Revision: https://phab.mercurial-scm.org/D5989

mercurial/copies.py

@@ -124,10 +124,13 b' def _chain(src, dst, a, b):'
             # file is a copy of an existing file
             t[k] = v
 
-    # remove criss-crossed copies
     for k, v in list(t.items()):
+        # remove criss-crossed copies
         if k in src and v in dst:
             del t[k]
+        # remove copies to files that were then removed
+        elif k not in dst:
+            del t[k]
 
     return t
 

tests/test-copies.t

@@ -194,9 +194,7 b' Fork renames x to y on one side and remo'
   |/   x y
   o  0 add x
      x
-BROKEN: x doesn't exist here
   $ hg debugpathcopies 1 2
-  y -> x
 
 Copies via null revision (there shouldn't be any)
   $ newrepo