Don't replace '\' with '\\' in before_prompt_hook

Don't replace '\' with '\\' in before_prompt_hook

In gdb/command/prompt.py:before_prompt_hook, the '\' in the new prompt
is replaced with '\\', shown 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 nor email.  As doc
said, "set extended-prompt \w" substitute the current working
directory, but it prints something different from what pwd or
os.getcwdu() prints on mingw32 host.

(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

This makes me think whether the substitution in before_prompt_hook is
necessary or not.  This patch is to remove this substitution.

Run gdb.python on x86_64-linux and arm-none-eabi on mingw32 host.  No
regressions.

gdb:

2014-10-30  Yao Qi  <yao@codesourcery.com>

	* python/lib/gdb/command/prompt.py (before_prompt_hook): Don't
	replace '\\' with '\\\\'.