Fixed core preparation in `branch.py` to remove all existing submodules
before creating new ones (necessary when using a `wmf/*` branch as a
branchpoint).

Fixed some issues with retrieval of skins/extensions from Gerrit where
collection repos such as `mediawiki/extensions` and `mediawiki/skins`
were being included as well as repos beneath the project structure of
extensions (e.g. `mediawiki/extensions/Parsoid/cpp/contrib/libhubbub`).

Added a `--core-bundle` to `branch.py` to allow for preparation of core
with a bundle of extensions/skins without performing branch creation on
those repos.

Fixed issues with core branch checkout and missing Gerrit commit-msg
hook.

Moved `branch.py` implementation to a module under the `mwrelease`
package so it could be more easily reused.

Finally, implemented a `branch-version.py` script that creates branches
and prepare core based on tips of the latest wmf branches running in
production. It takes a single position argument: a `maj.min.patch`
version (e.g. `1.34.0`).

Bug: T232024
Change-Id: I3e17366574f317766be291c7ee4a79f6da91947b