exchange: don't report failure from identical bookmarks b901645a8784 regressed the behavior of pushing an unchanged bookmark to a remote. Before that commit, pushing a unchanged bookmark would result in "exporting bookmark @" being printed. After that commit, we now see an incorrect message "bookmark %s does not exist on the local or remote repository!" This patch fixes the regression introduced by b901645a8784 by having the bookmark error reporting code filter identical bookmarks and adds a test for the behavior.