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 <ho...@cisco.com>
 git-rebase.sh     | 2 +-
 t/t3400-rebase.sh | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/git-rebase.sh b/git-rebase.sh
index b2f1c76..2c692c3 100755
--- a/git-rebase.sh
+++ b/git-rebase.sh
@@ -473,7 +473,7 @@ case "$#" in
                head_name="detached HEAD"
                branch_name=HEAD ;# detached
-       orig_head=$(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"
diff --git a/t/t3400-rebase.sh b/t/t3400-rebase.sh
index f6cc102..b58fa1a 100755
--- a/t/t3400-rebase.sh
+++ b/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 &&

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