Merge branch 'rt/maint-clone-single'

Running "git fetch" in a repository made with "git clone --single"
slurps all the branches, defeating the point of "--single".

* rt/maint-clone-single:
  clone --single: limit the fetch refspec to fetched branch

diff --cc Documentation/git-clone.txt
Simple merge

diff --cc builtin/clone.c
Simple merge