tests/test-revlog-group-emptyiter.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).

Issue1678: IndexError when pushing

setting up base repo
  $ hg init a
  $ cd a
  $ touch a
  $ hg ci -Am a
  adding a
  $ cd ..

cloning base repo
  $ hg clone a b
  updating to branch default
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ cd b

setting up cset to push
  $ hg up null
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ touch a
different msg so we get a clog new entry
  $ hg ci -Am b
  adding a
  created new head

pushing
  $ hg push -f ../a
  pushing to ../a
  searching for changes
  adding changesets
  adding manifests
  adding file changes
  added 1 changesets with 0 changes to 0 files (+1 heads)

  $ cd ..