bash_completion: don't complete export with "garbage" when mq is not around
The "garbage" is the output of hg help printed when a command is not found.
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
a
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a not present
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1 files, 1 changesets, 1 total revisions
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
a