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).
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