Teach appendShellString() to not quote strings containing "-".

Brain fade in commit a00c58314: I was thinking that a string starting with
"-" could be taken as a switch depending on command line syntax.  That's
true, but having appendShellString() quote it will not help, so we may as
well not do so.  Per complaint from Peter Eisentraut.

src/fe_utils/string_utils.c

@@ -439,7 +439,7 @@ appendShellString(PQExpBuffer buf, const char *str)
 	 * contains only safe characters.
 	 */
 	if (*str != '\0' &&
-		strspn(str, "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_./:") == strlen(str))
+		strspn(str, "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_./:") == strlen(str))
 	{
 		appendPQExpBufferStr(buf, str);
 		return;