Git Bash prompt doesn't work with set -u

[AnyOldName3]

• I was not able to find an open or closed issue matching what I'm seeing

Setup

• Which version of Git for Windows are you using? Is it 32-bit or 64-bit?

$ git --version --build-options

git version 2.28.0.windows.1
cpu: x86_64
built from commit: 77982caf269b7ee713a76da2bcf260c34d3bf7a7
sizeof-long: 4
sizeof-size_t: 8
shell-path: /bin/sh

• Which version of Windows are you running? Vista, 7, 8, 10? Is it 32-bit or 64-bit?

$ cmd.exe /c ver

Microsoft Windows [Version 10.0.18363.1016]

• What options did you set as part of the installation? Or did you choose the
  defaults?

# One of the following:
> type "C:\Program Files\Git\etc\install-options.txt"
> type "C:\Program Files (x86)\Git\etc\install-options.txt"
> type "%USERPROFILE%\AppData\Local\Programs\Git\etc\install-options.txt"
$ cat /etc/install-options.txt

Editor Option: Notepad++
Custom Editor Path:
Path Option: Cmd
SSH Option: OpenSSH
Tortoise Option: false
CURL Option: OpenSSL
CRLF Option: CRLFAlways
Bash Terminal Option: MinTTY
Git Pull Behavior Option: Merge
Use Credential Manager: Enabled
Performance Tweaks FSCache: Enabled
Enable Symlinks: Disabled
Enable Pseudo Console Support: Disabled

• Any other interesting things about your environment that might be related
  to the issue you're seeing?

Nope.

Details

• Which terminal/shell are you running Git from? e.g Bash/CMD/PowerShell/other

Git Bash

• What commands did you run to trigger this issue? If you can provide a
  Minimal, Complete, and Verifiable example
  this will help us understand the issue.

set -u
# the prompt will no longer display git information (e.g. the current branch), and after each command `bash: GIT_PS1_COMPRESSSPARSESTATE: unbound variable` will be printed

• What did you expect to occur after running these commands?

If I (or a script I ran) referred to unbound variables, an error message would be printed, but the git prompt would keep working and not reference unbound variables.

• What actually happened instead?

bash: GIT_PS1_COMPRESSSPARSESTATE: unbound variable

gets printed after each command

• If the problem was occurring with a specific repository, can you provide the
  URL to that repository to help us with testing?

N/A

More details, unconstrained by the issue report form

This only appeared with a recent update, so I had a look over the history, and determined that this commit is responsible: afda36d