tests: collect all branch creation in one place in test-copies-chain-merge.t
I found this test case really hard to read because it requires
scrolling back and forth between the setup and the verification. The
next patch will move the verification close to the merge commits they
test. This patch prepares for that by moving all branch creation first
so they are separate from the merge commits (because many merge
commits reuse the same branches).
Differential Revision: https://phab.mercurial-scm.org/D8376