shelve: choose a legal shelve name when no name is passed (issue5112) Currently if our branch name contains '\' or starts with '.', shelve chooses an illegal shelve name. This behaviour is not good as it itself is choosing something which it won't accept further. We can raise errors if user passes a name which is illegal. After this patch, if '\' is contained in branch name or bookmark name, it will be replaced by '_' while choosing a shelve name and if they starts with '.', the first '.' is replaced by '_'.

shelve: add tests to ensure illegal shelve names are avoided We avoid '.' as the first letter of shelve name so that we don't create hidden file. We also avoid slashes in name so that we don't form a new directory

py3: replace sys.executable with pycompat.sysexecutable sys.executable returns unicodes on Python 3. This patch replaces occurences of sys.executable with pycompat.sysexecutable.