Pass expected parameters to prepare_commit_msg

[samdeane]

When the git command line runs prepare-commit-hook, it is passed up to three parameters (see https://git-scm.com/docs/githooks#_prepare_commit_msg).

It looks like that's currently not happening when Fork runs it. It would be helpful if it did happen, so that existing hook scripts worked as expected.

Extra information, such as the existing "subject" and "description" field values, could also be passed as environment variables, eg: FORK_SUBJECT, and FORK_DESCRIPTION.

This would be helpful as it would allow the script to know that it was running from Fork.app, and modify its behaviour accordingly (if required).

[svoop]

Indeed, the way Fork uses the prepare-commit-hook is currently incompatible with CLI Git:

• Fork expects the script to echo the commit message template.
• Git passes the default commit message as a file with the first argument and expects the script to edit the contents of this file.

The second argument contains the source of commit, either message, template, merge, squash or commit (which requires a third argument). IMO, only the first and the last of these are relevant for Fork.

Furtunately, it's not very difficult to make things compatible. Fork would have to...

• [ ] Set the first argument to an empty, temporary file. The prepare-commit-hook then edits this file and exits, at which point Fork reads the temporary file and prefills the input with this content.
• [ ] If the amend checkbox is NOT selected: Set the second argument to message and not set any third argument.
• [ ] If the amend checkbox IS selected: Set the second argument to commit and the third argument to the SHA of the commit to be amended.

/cc @DanPristupov