checkout: do not get confused with ambiguous tag/branch names

checkout: do not get confused with ambiguous tag/branch names

Although it is not advisable, we have always allowed a branch
and a tag to have the same basename (i.e. it is not illegal to
have refs/heads/frotz and refs/tags/frotz at the same time).
When talking about a specific commit, the interpretation of
'frotz' has always been "use tag and then check branch",
although we warn when ambiguities exist.

However "git checkout $name" is defined to (1) first see if it
matches the branch name, and if so switch to that branch; (2)
otherwise it is an instruction to detach HEAD to point at the
commit named by $name.  We did not follow this definition when
$name appeared under both refs/heads/ and refs/tags/ -- we
switched to the branch but read the tree from the tagged commit,
which was utterly bogus.

Signed-off-by: Junio C Hamano <gitster@pobox.com>