The *-nightly git repo always uses the _master_ branch. The `index_only:`
support maintains a separate local branch since its git repo should be as
small as possible. The full repo should be maintained for mirrors not
using `index_only:` so that when it force-pushes, it does not need to always
push all the files, only the updated ones. So the full repo should be
maintained in the _master_ branch, and only the `index_only` mirrors should
have their own branch.
This adds a test case to reproduce this error:
https://gitlab.com/fdroid/fdroidclient/-/jobs/10347168516
This case also applies to any setup that used `servergitmirrors:` before the
`index_only:` feature was added. This also applies to cases if the repo
maintainer manually clones the *-nightly repo into _fdroid/git-mirror/_
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