bundle: introduce an higher level function to write bundle on disk The current function ('writebundle') is focussing on getting an existing changegroup to disk. It is no easy ways to includes more part in the generated bundle2. So we introduce a slightly higher level function that is fed the 'outgoing' object (that defines the bundled spec) and the bundlespec parameters (to control the changegroup generation and inclusion of other parts). This is creating the third logic dedicated to create a consistent bundle2 (the other 2 are the push code and the getbundle code). We should probably reconcile them at some points but they all takes different types of input. So we need to introduce an intermediate "object" that each different input could be converted to. Such unified "bundle2 specification" could be fed to some unified code. We start by having the `hg bundle` related code on its own to helps defines its specific needs first. Once the common and specific parts of each logic will be known we can start unification.

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