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.
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