git: Set "followremotehead" to "never" for fetches

https://lore.kernel.org/git/20240910203835.2288291-1-bence@ferdinandy.com/ was recently merged into the next branch resulting in some failures in our nightly tests.

Set the "followremotehead" option to revert to the previous behaviour so that the tests pass and we're not surprised by any changes during fetches. We'll also investigate whether we can just adopt the new behaviour rather than manually setting the local HEAD after a fetch.

#6546 is the follow-up issue.