Git: Support prepare-commit-msg hook

[Khady]

• VSCode Version: 1.43.2
• OS Version: ubuntu 19.10

In vscode, during a git commit, the prepare-commit-msg hook of git runs only after the message is already inputed by the user. It defeats the fact that this hook must prepare the message for the user.

The prepare-commit-msg hook is run before the commit message editor is fired up but after the default message is created.
https://git-scm.com/book/en/v2/Customizing-Git-Git-Hooks (emphasis is mine)

It means that the user can't benefit from the prepared message. For example, I have a hook which adds 10 lines of comments. Those comments contain some info that is helping me to write the final message. Ideally the prepare-commit-msg hook should run first, then the content created by the hook must go into the commit message input box, then the user can edit this text.

Apparently vscode is trying to commit by using an equivalent of git commit -m "the message", at least that is what I understand from the 2=message in step 12.

An additional note, it seems that if the message inputted by the user in vscode during a Git: Commit is # foo (so starting with a #), the hook doesn't run. That's... surprising?

Steps to Reproduce:

1. in a git repo create .git/hook/prepare-commit-msg with this content

echo "1=$1 2=$2 3=$3" > "$1"
date >> "$1"
cp "$1" /tmp/investigate-prepare-commit-msg

2. chmod +x .git/hook/prepare-commit-msg
3. run git commit --allow-empty and do not close the editor yet
4. check that the editor contains a message like

# some comment
1=.git/COMMIT_EDITMSG 2= 3=
Thu Apr  2 09:49:55 GMT 2020

5. check that /tmp/investigate-prepare-commit-msg contains the same message
6. close the editor and run rm /tmp/investigate-prepare-commit-msg
7. echo "creating a diff" > some_file && git add some_file && code .
8. in vscode, execute Git: Commit and do not input a message yet
9. check that the input box for the commit message is empty
10. check that /tmp/investigate-prepare-commit-msg does not exist
11. input a commit message and press enter
12. check that /tmp/investigate-prepare-commit-msg now exists and contains

# some comment
1=.git/COMMIT_EDITMSG 2=message 3=
Thu 02 Apr 2020 10:00:31 AM GMT

note the 2=message part
13. check that the commit contains the same thing as /tmp/investigate-prepare-commit-msg

Does this issue occur when all extensions are disabled?: Yes

[vscodebot]

(Experimental duplicate detection)
Thanks for submitting this issue. Please also check if it is already covered by an existing one, like:

• Git: Support editing the commit message in a text editor (#30562)

[vscodebot]

This feature request is now a candidate for our backlog. The community has 60 days to upvote the issue. If it receives 20 upvotes we will move it to our backlog. If not, we will close it. To learn more about how we handle feature requests, please see our documentation.

Happy Coding!

[Khady]

Will be fixed by #95266