Merge branch 'jc/no-git-config-in-clone'

We stopped paying attention to $GIT_CONFIG environment that points
at a single configuration file from any command other than "git config"
quite a while ago, but "git clone" internally set, exported, and
then unexported the variable during its operation unnecessarily.

* jc/no-git-config-in-clone:
  clone: do not export and unexport GIT_CONFIG

diff --cc builtin/clone.c
Simple merge