test-check-commit-hg: clarify misleading "commit message rules" error

The test case doesn't only check the commit message, but also the
patch, which can result in confusing output like

  +  Revision df6f06d17100 does not comply to commit message rules
  +  ------------------------------------------------------
  +  32: adds double empty line
  +
  +

even when there are no double blank lines in the commit message. Drop
the "commit message" part to make it less confusing.

tests/test-check-commit-hg.t

@@ -14,7 +14,7 b' Go back in the hg repo'
   $ for node in `hg log --rev 'draft() and ::.' --template '{node|short}\n'`; do
   >    hg export $node | contrib/check-commit > ${TESTTMP}/check-commit.out
   >    if [ $? -ne 0 ]; then
-  >        echo "Revision $node does not comply to commit message rules"
+  >        echo "Revision $node does not comply to rules"
   >        echo '------------------------------------------------------'
   >        cat ${TESTTMP}/check-commit.out
   >        echo