rebase: find orig_head unambiguously

When we 'git rebase $upstream', git uses 'rev-parse --verify
$current_branch' to find ORIG_HEAD.  But if $current_branch
is ambiguous, 'rev-parse --verify' emits a warning and returns
a SHA1 anyway.  When the wrong ambiguous choice is used,
git-rebase fails non-gracefully:  it emits a warning about
failing to lock $current_branch, an error about being unable to
checkout $current_branch again, and it might even decide the
rebase is a fast-forward when it is not.

In the 'rebase $upstream' case, we already know the unambiguous
spelling of $current_branch is "HEAD".  Fix git-rebase to find
$orig_head unambiguously.

Add a test in t3400-rebase.sh which creates an ambiguous branch
name and rebases it implicitly with 'git rebase $other'.

Signed-off-by: Phil Hord <hordp@cisco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -464,7 +464,7 @@ case "$#" in
 		head_name="detached HEAD"
 		branch_name=HEAD ;# detached
 	fi
-	orig_head=$(git rev-parse --verify "${branch_name}^0") || exit
+	orig_head=$(git rev-parse --verify HEAD) || exit
 	;;
 *)
 	die "BUG: unexpected number of arguments left to parse"

t/t3400-rebase.sh

@@ -101,7 +101,14 @@ test_expect_success 'HEAD was detached during rebase' '
 	test $(git rev-parse HEAD@{1}) != $(git rev-parse my-topic-branch@{1})
 '
 
+test_expect_success 'rebase from ambiguous branch name' '
+	git checkout -b topic side &&
+	git rebase master
+'
+
 test_expect_success 'rebase after merge master' '
+	git checkout --detach refs/tags/topic &&
+	git branch -D topic &&
 	git reset --hard topic &&
 	git merge master &&
 	git rebase master &&