bookmarks: do not forward merged bookmark (issue1877)

tests/test-issue1877.t

@@ -0,0 +1,45 b''
+http://mercurial.selenic.com/bts/issue1877
+
+  $ hg init a
+  $ cd a
+  $ echo a > a
+  $ hg add a
+  $ hg ci -m 'a'
+  $ echo b > a
+  $ hg ci -m'b'
+  $ hg up 0
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg book main
+  $ hg book
+   * main                      0:cb9a9f314b8b
+  $ echo c > c
+  $ hg add c
+  $ hg ci -m'c'
+  created new head
+  $ hg book
+   * main                      2:d36c0562f908
+  $ hg heads
+  changeset:   2:d36c0562f908
+  bookmark:    main
+  tag:         tip
+  parent:      0:cb9a9f314b8b
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     c
+  
+  changeset:   1:1e6c11564562
+  user:        test
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  summary:     b
+  
+  $ hg up 1e6c11564562
+  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ hg merge main
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ hg book
+     main                      2:d36c0562f908
+  $ hg ci -m'merge'
+  $ hg book
+     main                      2:d36c0562f908
+

mercurial/localrepo.py

@@ -1024,10 +1024,7 b' class localrepository(repo.repository):'
                 raise
 
             # update bookmarks, dirstate and mergestate
-            parents = (p1, p2)
-            if p2 == nullid:
-                parents = (p1,)
-            bookmarks.update(self, parents, ret)
+            bookmarks.update(self, p1, ret)
             for f in changes[0] + changes[1]:
                 self.dirstate.normal(f)
             for f in changes[2]: