bug: git branch -d and case-insensitive file-systems

I use git (2.2.1) on OS X (10.9.5) and recently my repo got into a bad state. I think this involves a mis-handling of case-insensitive file systems.

This reproduces the problem:

    > git init
    Initialized empty Git repository in /Users/aarond_local/code/git-test/.git/
    > git commit --allow-empty -m 'first commit'
    [master (root-commit) 923d8b8] first commit
    > git checkout -b feature
    Switched to a new branch 'feature'
    > git checkout -b Feature
    fatal: A branch named 'Feature' already exists.
    > git checkout -B Feature
    Switched to and reset branch 'Feature'
    > git branch -d feature
    Deleted branch feature (was 923d8b8).
    > git log
    fatal: bad default revision 'HEAD'

This is the behavior when there isn't a case mismatch, which is what I would have expected in the previous case as well:

    > git init
    Initialized empty Git repository in /Users/aarond_local/code/git-test/.git/
    > git commit --allow-empty -m 'first commit'
    [master (root-commit) 48df19f] first commit
    > git checkout -b feature
    Switched to a new branch 'feature'
    > git branch -d feature
    error: Cannot delete the branch 'feature' which you are currently on.

I can also reproduce the issue on git 2.5.0.

-Aaron Dufour