Substitute '\' with '\\' in extended-prompt

[Yao Qi <yao@codesourcery.com>]

Hi Phil,
Why do we do the substitute in extended-prompt in your extended-prompt
patch <https://sourceware.org/ml/gdb-patches/2011-08/msg00236.html> as
below?

> +    def before_prompt_hook(self, current):
> +        if self.value is not '':
> +            newprompt = gdb.prompt.substitute_prompt(self.value)
> +            return newprompt.replace('\\', '\\\\')
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +        else:
> +            return None

I don't see any explanations on this in comments or email.

Such substitute makes the output of "set extended-prompt \w" different
from the output of "pwd" nor "os.getcwdu()"

(gdb) python print os.getcwdu()^M
\\build2-lucid-cs\yqi\yqi\arm-none-eabi

(gdb) pwd^M
Working directory \\build2-lucid-cs\yqi\yqi\arm-none-eabi

(gdb) set extended-prompt \w
\\\\build2-lucid-cs\\yqi\\yqi\\arm-none-eabi

My patch <https://sourceware.org/ml/gdb-patches/2014-10/msg00423.html>
doesn't work due to this difference.  I'd like to know whether the
substitute is necessary or not.

-- 
Yao (齐尧)