rebase: check that the bookmark is still valid when restoring (issue4669)

After a rebase --abort, we attempt to restore the previously active
bookmark.  We need to ensure that the bookmark still exists.

hgext/rebase.py

@@ -892,7 +892,7 b' def abort(repo, originalwd, target, stat'
             # no backup of rebased cset versions needed
             repair.strip(repo.ui, repo, strippoints)
 
-    if activebookmark:
+    if activebookmark and activebookmark in repo._bookmarks:
         bookmarks.setcurrent(repo, activebookmark)
 
     clearstatus(repo)

tests/test-bookmarks-rebase.t

@@ -90,3 +90,18 b' aborted rebase should restore active boo'
    * three                     4:dd7c838e8362
      two                       3:42e5ed2cdcf4
 
+after aborted rebase, restoring a bookmark that has been removed should not fail
+
+  $ hg rebase -s three -d two
+  rebasing 4:dd7c838e8362 "4" (tip three)
+  merging d
+  warning: conflicts during merge.
+  merging d incomplete! (edit conflicts, then use 'hg resolve --mark')
+  unresolved conflicts (see hg resolve, then hg rebase --continue)
+  [1]
+  $ hg bookmark -d three
+  $ hg rebase --abort
+  rebase aborted
+  $ hg bookmark
+     one                       1:925d80f479bb
+     two                       3:42e5ed2cdcf4