tests: fix test-check-commit.t when all commits are public

I'm 99% sure this is a portable use of /bin/[, and it seems to fix the
issue I noticed on the buildbot on my machine.

tests/test-check-commit.t

@@ -8,10 +8,13 Go back in the hg repo
 
   $ cd $TESTDIR/..
 
+  $ REVSET='not public() and ::. and not desc("# no-check-commit")'
+
   $ mkdir "$TESTTMP/p"
-  $ testrepohg export --git -o "$TESTTMP/p/%n-%h" \
-  > -r 'not public() and ::. and not desc("# no-check-commit")'
-  $ for f in `ls "$TESTTMP/p"`; do
+  $ REVS=`testrepohg log -r "$REVSET" -T.`
+  $ if [ -n "$REVS" ] ; then
+  >   testrepohg export --git -o "$TESTTMP/p/%n-%h" -r "$REVSET"
+  >   for f in `ls "$TESTTMP/p"`; do
   >    contrib/check-commit < "$TESTTMP/p/$f" > "$TESTTMP/check-commit.out"
   >    if [ $? -ne 0 ]; then
   >        node="${f##*-}"
@@ -21,3 +24,4 Go back in the hg repo
   >        echo
   >   fi
   > done
+  > fi