Pkg: be crystal clear about the branch location

Prevents the following error:

julia> Pkg.update()
warning: refname 'origin/devel' is ambiguous.
fatal: Ambiguous object name: 'origin/devel'.
ERROR: failed process: Process(`git branch -f devel origin/devel`, ProcessExited(128)) [128]
 in error at error.jl:22
 in pipeline_error at process.jl:394
 in run at process.jl:384
 in anonymous at no file:567
 in cd at file.jl:25
 in anonymous at no file:564
 in cd at file.jl:25
 in cd_pkgdir at pkg.jl:42
 in update at pkg.jl:563