bookmarks: reuse @number bookmark, if it refers changeset referred remotely
Before this patch, "@number" suffixed bookmark may be newly created at
each "hg pull" from the remote repository, if the bookmark in remote
repository diverges from one in local one.
This causes unexpected increase of "@number" suffixed bookmarks.
This patch reuses "@number" suffixed bookmark, if it refers the
changeset which is referred by the same bookmark in the remote
repository.