Confusing treatment of "head" in worktree on case-insensitive FS

Hello,

When I run `git rev-parse head` inside a worktree on a case insensitive filesystem, the result is different from `git rev-parse HEAD`.

Steps to reproduce, on a case insensitive filesystem:

$ git init $ git commit --allow-empty -m'test' $ git worktree add /tmp/myworktree $ cd /tmp/myworktree $ git commit --allow-empty -m'test' $ git rev-parse head adf59ca8da0ee5c4eb455f87efecc6c79eaf030f $ git rev-parse hEAd adf59ca8da0ee5c4eb455f87efecc6c79eaf030f $ git rev-parse HEAD fbb28196d08d74aa61f65e5cee3cb11cc24c338a

This also happens when using any other command involving "head", like `git show head`, `git diff head`, etc.

What did you expect to happen?

If `git rev-parse head` returns anything at all (of course on a case-sensitive filesystem it just returns an error), I'd expect it to be the same as `git rev-parse HEAD`.

I accidentally got into the happen of using `git diff head` instead of `git diff HEAD` on a case insensitive filesystem and was thrown off by this when I ran into it.

What happened instead?

When in a worktree, `head` appears to get resolved to the value of `HEAD` in `git rev-parse --git-common-dir`, while `HEAD` gets resolved to the value of `HEAD` in the worktree.

[System Info] git version: git version 2.39.2 (Apple Git-143) cpu: arm64 no commit associated with this build sizeof-long: 8 sizeof-size_t: 8 shell-path: /bin/sh feature: fsmonitor--daemon uname: Darwin 23.4.0 Darwin Kernel Version 23.4.0: Fri Mar 15 00:12:41 PDT 2024; root:xnu-10063.101.17~1/RELEASE_ARM64_T8103 arm64 compiler info: clang: 14.0.3 (clang-1403.0.22.14.1) libc info: no libc information available $SHELL (typically, interactive shell): /usr/bin/fish