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>

git-checkout.sh

@@ -63,12 +63,13 @@ while [ "$#" != "0" ]; do
 				echo "unknown flag $arg"
 				exit 1
 			fi
-			new="$rev"
 			new_name="$arg"
 			if git-show-ref --verify --quiet -- "refs/heads/$arg"
 			then
+				rev=$(git-rev-parse --verify "refs/heads/$arg^0")
 				branch="$arg"
 			fi
+			new="$rev"
 		elif rev=$(git-rev-parse --verify "$arg^{tree}" 2>/dev/null)
 		then
 			# checking out selected paths from a tree-ish.