merge: drop underscore prefix from _checkunknown()
The method has been called from commands.py since
3eab42088be4
(update: just merge unknown file collisions, 2012-02-09), so drop the
underscore prefix that suggests that it's private.
$ echo 'raise Exception("bit bucket overflow")' > badext.py
$ abspath=`pwd`/badext.py
$ cat <<EOF >> $HGRCPATH
> [extensions]
> gpg =
> hgext.gpg =
> badext = $abspath
> badext2 =
> EOF
$ 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
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