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.