Merge branch 'ab/fetch-set-upstream-while-detached'

"git fetch --set-upstream" did not check if there is a current
branch, leading to a segfault when it is run on a detached HEAD,
which has been corrected.

* ab/fetch-set-upstream-while-detached:
  pull, fetch: fix segfault in --set-upstream option

diff --cc builtin/fetch.c
Simple merge