bookmarks: fix head selection for merge with two bookmarked heads

A type mismatch caused the search for the other head to fail. The code is
fragile, and instead it ended up using the 'first' bookmark head, but the
ordering is undefined and it could thus randomly use the wrong bookmarkhead
and fail with:

  $ hg up -q -C e@diverged
  $ hg merge
  abort: merging with a working directory ancestor has no effect

mercurial/commands.py

@@ -4288,7 +4288,7 b' def merge(ui, repo, node=None, **opts):'
 
     if not node and repo._bookmarkcurrent:
         bmheads = repo.bookmarkheads(repo._bookmarkcurrent)
-        curhead = repo[repo._bookmarkcurrent]
+        curhead = repo[repo._bookmarkcurrent].node()
         if len(bmheads) == 2:
             if curhead == bmheads[0]:
                 node = bmheads[1]

tests/test-bookmarks-merge.t

@@ -75,9 +75,27 b''
   $ echo f > f
   $ hg commit -Am "f"
   adding f
+  $ hg bookmarks -r 4 "e@diverged"
+  $ hg up -q -C "e@diverged"
+  $ hg merge
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg parents
+  changeset:   4:a0546fcfe0fb
+  bookmark:    e@diverged
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     d
+  
+  changeset:   5:26bee9c5bcf3
+  bookmark:    e
+  parent:      3:b8f96cf4688b
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     e
+  
   $ hg up -C e
-  1 files updated, 0 files merged, 2 files removed, 0 files unresolved
-  $ hg bookmarks -r 4 "e@diverged"
+  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
   $ hg bookmarks
      b                         1:d2ae7f538514
      c                         3:b8f96cf4688b