git-gui: fix GIT_DIR handling for submodules.

[oldium]

When dealing with submodule, the GIT_DIR might not be set. If it is not set
before, it cannot be unset and the execution ends with error:

can't unset "env(GIT_DIR)": no such element in array

Fix this by checking if GIT_DIR can be unset.

[dscho]

This PR looks good to me, but in the meantime Git GUI has an active maintainer again, so may I ask you to rebase this and open a GitGitGadget PR? I.e.

1. git remote add gitgitgadget https://github.com/gitgitgadget/git
2. git fetch gitgitgadget git-gui/master
3. git branch --set-upstream-to=gitgitgadget/git-gui/master
4. git rebase --onto @{u} HEAD^
5. force-push
6. open a PR at https://github.com/gitgitgadget/git
7. ping me (or other active people) in the PR to get permission to use GitGitGadget
8. add a single comment /submit to send the patch off to the Git mailing list, Cc:ing the Git GUI maintainer

[dscho]

Git for Windows has different code for Git GUI, so this fix is specifically for Git for Windows. The main difference is commit ca2f932, which is not present in mainline Git

Oh, that's a good point!

(and I actually do not understand why it is there).

Right. I did stumble over it a couple times, and once I tried to figure out what was the rationale, but also failed.

Maybe we should just revert the revert instead, i.e. drop it from Git for Windows' patches?

[oldium]

Yes, drop ca2f932 together with a9af64e.

[dscho]

Yes, drop ca2f932 together with a9af64e.

I found the reason for ca2f932! Look at https://groups.google.com/d/msg/msysgit/FkdTopndAes/RXF_h8eAgW4J

[dscho]

Apparently, this failed (in a worktree with an initialized submodule):

git gui
# the following instructions are for clicking

• Tools->Add...
  Enter Name: submodule update
  Enter Command: git submodule update
• Add
• Tools->submodule update

Error message is:
"fatal: reference is not a tree: 543f8d6
"Unable to checkout '543f8d61eb235820aecefde6e46ec90e9803a6d8' in submodule path 'doc/git/html'"

@oldium you probably have a worktree with initialized submodule(s), right? Could you try whether the indicated bug is still there after reverting a9af64e and ca2f932?

[oldium]

Yes, I do. I will play with this a little bit.

[dscho]

@oldium thank you.

I dug a little further, and it seems that https://groups.google.com/d/msg/msysgit/FkdTopndAes/BRVBc95idk8J was intended to (quote:) "fix this properly", and that patch made it into Git's master via the merge bd08ecc (the patch in question is 74ae141).

So yes, it should be possible for us to just drop those two commits.

If your testing confirms this, would you terribly mind updating this here PR accordingly? It is unfortunately too late for v2.25.0-rc1 (which is already in the process of being released), but it will make it into -rc2 at least.

[oldium]

I tried the following which I found in the mailing list:

git clone git://repo.or.cz/msysgit.git
cd msysgit
git submodule update --init
git gui
# the following instructions are for clicking
* Tools->Add...
Enter Name: submodule update
Enter Command: git submodule update
* Add
* Tools->submodule update

Result:

[dscho]

Perfect! So we can drop this revert after slightly less than 10 years ;-)

[oldium]

Updated PR.

[dscho]

Thank you @oldium!

[oldium]

You are welcome, @dscho!