tests: avoid renamed files triggering check-commit failures

I had to flag some revisions to not undergo check-commit scrutiny
today to get the tests to pass, because they renamed files that had
existing failures which we're not cleaning up. Let's prevent that in
the future by using the extended git diff format to elide moved code.

tests/test-check-commit.t

@@ -9,7 +9,7 b' Go back in the hg repo'
   $ cd $TESTDIR/..
 
   $ for node in `hg log --rev 'not public() and ::. and not desc("# no-check-commit")' --template '{node|short}\n'`; do
-  >    hg export $node | contrib/check-commit > ${TESTTMP}/check-commit.out
+  >    hg export --git $node | contrib/check-commit > ${TESTTMP}/check-commit.out
   >    if [ $? -ne 0 ]; then
   >        echo "Revision $node does not comply with rules"
   >        echo '------------------------------------------------------'