git-commit-font-lock-keywords-1 is static despite containing "custom" members

`git-commit-known-pseudo-headers' is supposed to be customizable, but its value does not get used because`git-commit-font-lock-keywords-1` is evaluated before customization.

I discovered this while adding a `git-commit-summary-line-length-limit`, which cannot work until `git-commit-font-lock-keywords-1` becomes more dynamic.
