The mergebasebranch option does the base branch merge too late

[Note that this problem predates the mergebasebranch option, since we did the same (but only on Jenkins) before]

Right now when we run rake build in a non-base branch it will be run at the Git state before the base branch merge, resulting in mixed Git state. And since the merge happens in auto/build it will obviously also be run in an pre-merge state.

The cleanest solution I can think of is:

• remove the mergebasebranch option and the base branch merge code in auto/build
• introduce a Rake task prepare-ci-build that does the base branch merge
• make jenkins build with: rake prepare-ci-build && rake build

This way we limit this problem to only prepare-ci-build, which should be so simple that it essentially never will change.

Related issues

• Related to #14459 (closed)

Original created by @anonym on 12557 (Redmine)