make git-commit-known-pseudo-headers customizable

Make the previously constant `git-commit-known-pseudo-headers'
customizable.  Replace the variable `git-commit-mode-font-lock-keywords'
with a function by the same name.

Tested-by: Jonas Bernoulli <jonas@bernoul.li>
add "Suggested-by" pseudo commit message header

Suggested-by: Myself I think <jonas@bernoul.li>
move git-commit-mode-map into its own top-level section
add new top-level "Font-Lock" section

"Move" everything related to font-locking into new top-level "Font-Lock"
section.  This includes all the header-related font-locking previously
located in the "Headers" section and the only function that was actually
moved: `git-commit-font-lock-diff'.  According to Git what was moved is
the unrelated `git-commit-mode-syntax-table' but that is due to patch
minimizing gone wrong.
plug eyelashes

Aka "align stuff for purely cosmetic reasons".