Allow for further development on the main branch between pre_release() and CRAN acceptance

[maelle]

Maybe by splitting pre_release().

[krlmlr]

We discussed protecting the main branch after pre_release(), but I no longer think this is necessary. Instead, I now believe it's sufficient to slightly alter the behavior of bump_version("dev") if a release branch exists.

• We no longer call bump_version("dev") in pre_release() .
• Accepting a release would consist of merging the release branch into the main branch, and then bump_version("dev") .
• We need to take special care to ensure that the release branch can be merged back into the main branch without conflicts even after bump_version("dev") on the main branch. This now is ensured by bumping before branching off, but I think it can be made work also if we defer the bumping.

[maelle]

Is this actually a duplicate of #59 :thinking:

[krlmlr]

Doesn't look like it is, #59 can happen anyway.

Taking this on now.

[krlmlr]

Now in #614.