Karel Koci authored Jul 13, 2021 and Josef Schlehofer committed Jul 16, 2021

The master is not default for all repositories and there are some
existing repositories that migrated to the different default branch
name. The git provides instead generic alias HEAD for current branch
that on remote is resolved to the default branch and thus to master or
what it is called in that project. The issue is that while HEAD works
when we fetch we can't use it for checkout because it references the
local HEAD in that case. We have to use FETCH_HEAD instead as that is
the reference git gives to head fetched from the remote server.