copies: clarify which case some conditional are handling

This make the function a bit clearer. The middle conditional get no label
because we about about to remove it. See next changeset for details.

Differential Revision: https://phab.mercurial-scm.org/D9794

mercurial/copies.py

@@ -60,13 +60,13 b' def _filter(src, dst, t):'
 
     for k, v in list(t.items()):
         # remove copies from files that didn't exist
-        if v not in src:
+        if v not in src:  # case 5
             del t[k]
         # remove criss-crossed copies
         elif k in src and v in dst:
             del t[k]
         # remove copies to files that were then removed
-        elif k not in dst:
+        elif k not in dst:  # case 1
             del t[k]