Changes needed because `git-commit.el` is no longer distributed as a separate package

[tarsius]

Since Magit v4.1.0, git-commit.el is no longer distributed as a separate package git-commit. It is now part of the magit package.

That means that this package can no longer depend on git-commit in the Package-Requires library header and instead has to depend on magit.

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I had a quick look at how git-commit.el is used in this package and noticed that it probably wasn't compatible anymore, even before this recent change. It appears that this package operates under the assumption that git-commit-mode is a major-mode, when in fact it is a minor-mode, and has been for many years.

Looking through the issue tracker, I also seem many problems that have gone unaddressed and which I would assume render this package unusable. Since I also help maintain Melpa, I wonder whether it would be appropriate to remove this from Melpa.