Merge branch 'rt/maint-clone-single' into maint

A repository created with "git clone --single" had its fetch
refspecs set up just like a clone without "--single", leading the
subsequent "git fetch" to slurp all the other branches, defeating
the whole point of specifying "only this branch".

* 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