mq: do not call [0] on revset
The __getitem__ method have been removed. The "first" method is to be used
instead. Test have been extended to test this code path.
$ echo 'raise Exception("bit bucket overflow")' > badext.py
$ abspath=`pwd`/badext.py
$ echo '[extensions]' >> $HGRCPATH
$ echo "gpg =" >> $HGRCPATH
$ echo "hgext.gpg =" >> $HGRCPATH
$ echo "badext = $abspath" >> $HGRCPATH
$ echo "badext2 =" >> $HGRCPATH
$ hg -q help help
*** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
*** failed to import extension badext2: No module named badext2
hg help [-ec] [TOPIC]
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