merge: break out "both renamed a -> b" case

We can further limit the scope of the 2-way merge case by breaking out
the case where the file was not created from scratch on both sides but
rather renamed in the same way (and is therefore a 3-way merge). This
involves copying some code, but it makes it clearer which case the
"Note:" in the code refers to.

mercurial/merge.py

@@ -416,10 +416,14 def manifestmerge(repo, wctx, p2, pa, br
             continue
         if n1 and n2:
             if f not in ma:
-                # Note: f as ancestor is wrong - we can't really make a 3-way
-                # merge without an ancestor file.
-                fa = copy.get(f, f)
+                fa = copy.get(f, None)
+                if fa is not None:
                 actions['m'].append((f, (f, f, fa, False, pa.node()),
+                                   "both renamed from " + fa))
+                else:
+                    # Note: f as ancestor is wrong - we can't really make a
+                    # 3-way merge without an ancestor file.
+                    actions['m'].append((f, (f, f, f, False, pa.node()),
                                "both created"))
             else:
                 a = ma[f]

tests/test-rename-merge2.t

@@ -343,7 +343,7 args:
    ancestor: 924404dff337, local: 62e7bf090eba+, remote: 49b6d8032493
    preserving b for resolve of b
    preserving rev for resolve of rev
-   b: both created -> m
+   b: both renamed from a -> m
   updating: b 1/2 files (50.00%)
   picked tool 'python ../merge' for b (binary False symlink False)
   merging b