tests/test-check-commit-hg.t

posix: quote the specified string only when it may have to be quoted

This patch makes "posix.shellquote" examine the specified string and
quote it only when it may have to be quoted for safety, like as the
previous patch for "windows.shellquote".

In fact, on POSIX environment, quoting itself doesn't cause issues
like issue4463. But (almost) equivalent quoting policy can avoid
examining test result differently on POSIX and Windows (even though
showing command line with "%r" causes such examination in
"test-extdiff.t").

The last hunk for "test-extdiff.t" in this patch isn't needed for the
previous patch for "windows.shellquote", because the code path of it
is executed only "#if execbit" (= avoided on Windows).

#require test-repo

Enable obsolescence to avoid the warning issue when obsmarker are found

  $ cat >> $HGRCPATH << EOF
  > [experimental]
  > evolution=createmarkers
  > EOF

Go back in the hg repo

  $ cd $TESTDIR/..

  $ 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 rules"
  >        echo '------------------------------------------------------'
  >        cat ${TESTTMP}/check-commit.out
  >        echo
  >   fi
  > done