git difftool --no-index option is not respected ("not a git repository" error) when outside a repo

[logan-pugh]

• 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 2.21.0.windows.1
cpu: x86_64
built from commit: 2481c4cbe949856f270a3ee80c802f5dd89381aa
sizeof-long: 4
sizeof-size_t: 8

• 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 10.0.17763.316]

• 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: VIM
Custom Editor Path:
Path Option: BashOnly
SSH Option: OpenSSH
CURL Option: OpenSSL
CRLF Option: CRLFCommitAsIs
Bash Terminal Option: MinTTY
Performance Tweaks FSCache: Enabled
Use Credential Manager: Disabled
Enable Symlinks: Disabled

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

I was not able to reproduce the issue on a Linux VM, so I am posting this here on the Git for Windows issue tracker.

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.

user@machine MINGW64 ~
$ touch a.txt

user@machine MINGW64 ~
$ touch b.txt

user@machine MINGW64 ~
$ git diff --no-index a.txt b.txt

user@machine MINGW64 ~
$ git difftool --no-index a.txt b.txt
fatal: not a git repository (or any of the parent directories): .git

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

I would have expected the git difftool output to match the git diff output.

• What actually happened instead?

The error fatal: not a git repository (or any of the parent directories): .git was returned.

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

N/A (this occurs when current directory is outside of any Git repository)

[dscho]

Thank you for your bug report. As you can see, I already put up a Pull Request, and after leaving the good core Git reviewers an appropriate time to look over this (and possibly help me improve it), I will merge it and ask you to validate the fix.

[logan-pugh]

@dscho Much appreciated. I didn't think to check the Git version on my Linux VM, turns out it is quite old indeed (2.8.1). I will follow the PR as well. Thanks!

[dscho]

So the reviews are coming along nicely, but @gitster has not picked up the patches yet, so I have no idea whether major changes will be needed. I'd rather not integrate patches into Git for Windows unless I am relatively certain that they will make it into upstream Git (at least mostly) unchanged...