run-tests: fix _checkhglib() so it's correct when using --with-hg.
Since this makes the warning less likely, upgrade it to a "real"
warning to stderr.
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adding qqq.txt
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test 1:16b24da7e457
test2 1:16b24da7e457
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saving bundle to
saving bundle to
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* test 1:9f1b7e78eff8
* test2 1:9f1b7e78eff8