git-gui: create a new namespace for chord script evaluation

git-gui: create a new namespace for chord script evaluation

Evaluating the script in the same namespace as the chord itself creates
potential for variable name collision. And in that case the script would
unknowingly use the chord's variables.

For example, say the script has a variable called 'is_completed', which
also exists in the chord's namespace. The script then calls 'eval' and
sets 'is_completed' to 1 thinking it is setting its own variable,
completely unaware of how the chord works behind the scenes. This leads
to the chord never actually executing because it sees 'is_completed' as
true and thinks it has already completed.

Avoid the potential collision by creating a separate namespace for the
script that is a child of the chord's namespace.

Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>