Merge branch 'jk/allow-fetch-onelevel-refname' into maint

"git clone" would fail to clone from a repository that has a ref
directly under "refs/", e.g. "refs/stash", because different
validation paths do different things on such a refname.  Loosen the
client side's validation to allow such a ref.

* jk/allow-fetch-onelevel-refname:
fetch-pack: do not filter out one-level refs

diff --cc fetch-pack.c
Simple merge

diff --cc t/t5510-fetch.sh
Simple merge