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

[jedbrown]

git-commit-known-pseudo-headers' is supposed to be customizable, but its value does not get used becausegit-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.