git pull --rebase problems.

[Roland McGrath]

> >> error: 'refs/remotes/origin/hjl/pr14557' exists; cannot create
> >> 'refs/remotes/origin/hjl/pr14557/map'
[...]
> What causes this to happen?
> 
> Deleting and recreating remote branches with the same names?

As the error message indicates, there was previously a branch called
hjl/pr14557.  That translates (sometimes) to a (plain-text) file called
.git/refs/.../hjl/pr14557 existing.  Now there is a branch called
hjl/pr14557/map, which translates (sometimes) to a file called
.git/refs/.../hjl/pr14557/map needing to exist.  That implies that
a directory called .git/refs/.../hjl/pr14557 needs to be created,
but there is already a non-directory file by that name.

> I don't like having to obey git without knowing what it's doing...

Nobody ever knows exactly what git is doing.