Edit: This has been fixed now.
After having browsed through a number of review threads I noticed that very many of them received the following comment:
It appears you are working in the "master" branch in git. You should really be working in a version specific branch. The most direct documentation on this is Moving from a master branch to a version branch. For additional resources please see the documentation about release naming conventions and creating a branch in git.
The reason for this common problem is blatantly obvious. While the page creating a branch in git is correct, the instructions that is shown when you navigate to Your projects ➔ Add a new project ➔ [fill in form] ➔ Version control is outdated, as should be evident from the illustration below:
Some time ago, I posted a request for correction/clarification in the documentation issue queue. In the one and a half year that has passed, there has been absolutely no response on that issue, so I guess the documentation crowd is not interested.
I am now posting this in the Code Review of Full Project Applications Group, since feeding developers wrong information about how they go about setting up their empty sandboxes introduces one extra error into what we're handed for review. What I hope for is that someone in this group with the appropriate access level and/or "connections" shows an interest in getting this fixed.