bookmarks: avoid traceback when two pushes race to delete the same bookmark

`hg push -f -B remote-only-bookmark` can raise server-side in
`bookmarks._del` (specifically in `self._refmap.pop(mark)`), if the
remote-only bookmark got deleted concurrently.

Fix this by simply not deleting the non-existent bookmark in that
case.

For avoidance of doubt, refusing to delete a bookmark that doesn't
exist when the push starts is taking care of elsewhere; no change of
behavior there.

Differential Revision: https://phab.mercurial-scm.org/D8124

mercurial/bookmarks.py

@@ -173,6 +173,8 b' class bmstore(object):'
             nrefs.sort()
 
     def _del(self, mark):
+        if mark not in self._refmap:
+            return
         self._clean = False
         node = self._refmap.pop(mark)
         nrefs = self._nodemap[node]

relnotes/next

@@ -27,6 +27,7 b''
 
 == Bug Fixes  ==
 
+ * Fix server exception when concurrent pushes delete the same bookmark
 
 == Backwards Compatibility Changes ==