tests: add magic string to bypass check-commit

Allow bypassing test-check-commit runs by specifying '# no-check-commit'
in the description. This should be avoided but is useful for upstream
imports such as pywatchman which will cause check-code to fail otherwise.

tests/test-check-commit.t

@@ -8,7 +8,7 b' Go back in the hg repo'
 
   $ cd $TESTDIR/..
 
-  $ for node in `hg log --rev 'not public() and ::.' --template '{node|short}\n'`; do
+  $ 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
   >    if [ $? -ne 0 ]; then
   >        echo "Revision $node does not comply with rules"