Merge branch 'jn/remote-helpers-with-git-dir' into maint

"git ls-remote" and "git archive --remote" are designed to work
without being in a directory under Git's control.  However, recent
updates revealed that we randomly look into a directory called
.git/ without actually doing necessary set-up when working in a
repository.  Stop doing so.

* jn/remote-helpers-with-git-dir:
remote helpers: avoid blind fall-back to ".git" when setting GIT_DIR
remote: avoid reading $GIT_DIR config in non-repo

diff --cc remote.c
Simple merge

diff --cc t/t5550-http-fetch-dumb.sh
Simple merge

diff --cc transport-helper.c
Simple merge