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 '_'.
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