[1.8.0] git checkout refs/heads/foo checks out branch foo

Proposal:

'git checkout refs/heads/foo' (or 'git checkout heads/foo' for that
matter) does not check out the branch, but instead detaches HEAD at
foo. This is quite counter-intuitive (at least to me) and the same
functionality can be achieved by using e.g. foo~0. Change the behavior
so that the branch is actually checked out. This also applies to
e.g. 'git rebase master refs/heads/topic', which currently rebases a
detached HEAD. There are probably other examples as well that I'm not
aware of.


Risks:

Existing scripts may depend on the current behavior. It seems unlikely
that many users depend on it. Most likely, they use foo~0 or foo^0
instead.


Migration plan:

Make 'git checkout refs/head/foo' emit a warning in the next 1.7.x
explaining that its semantics will change in 1.8.0. Then change the
behavior in 1.8.0 and remove the warning.