support git refs in @ syntax

[zzzeek]

We'd like to be able to put paths to gerrit reviews in requirements files.

Given a gerrit like https://review.openstack.org/#/c/345601/6, the path given for a git pull looks like:

git pull https://git.openstack.org/openstack/oslo.db refs/changes/01/345601/6

pip syntax we'd expect would be:

.venv/bin/pip install -e git+https://git.openstack.org/openstack/oslo.db@refs/changes/01/345601/6#egg=oslo.db

current output:

Obtaining oslo.db from git+https://git.openstack.org/openstack/oslo.db@refs/changes/01/345601/6#egg=oslo.db
  Cloning https://git.openstack.org/openstack/oslo.db (to refs/changes/01/345601/6) to ./.venv/src/oslo.db
  Could not find a tag or branch 'refs/changes/01/345601/6', assuming commit.
error: pathspec 'refs/changes/01/345601/6' did not match any file(s) known to git.
Command "git checkout -q refs/changes/01/345601/6" failed with error code 1 in /home/classic/.venv/src/oslo.db

[pradyunsg]

@sshnaidm Please use Github reactions. They're meant for exactly this.