commitctx: move copy meta config reading in a dedicated function The logic is non trivial, make it contained in a function is clearer. It also unlock easy re-use of that logic without having the pass the value around.

commitctx: no longer use the `writecopiesto` variable in the function The `writefilecopymeta` variable already carry the same information, so we can use `writefilecopymeta` in the one conditional where `writecopiesto` was used.

commitctx: extract the function that commit a new manifest The logic is large enough and isolated enough to be extracted, this reduce the size of the main function, making it simpler to follow.

commitctx: stop using weakref proxy for transaction This weakref proxy was introduced in 2007 by 30d4d8985dd8. If I understand it correctly, the logic at that time was relying on the transaction destructor, triggered at garbage collection time to rollback failed transaction. passing the object to sub function directly mean it would live in the function scope and be trapped in the traceback on exception, leading to the transaction rollback too late in some case. Modern transaction usage use explicit opening and closing of transaction and no longer rely on some internal reference counting details. So this weakref proxy is no longer necessary. Absolutely no test are affected when we drop it.