In the status buffer:

Key	Description / Command	Mnemonic / Note
c c	commit staged changes	commit
	magit-commit-create
c w	reword last commit message	word
	magit-commit-reword	rewrites history
c e	add staged changes to last commit	extend
	magit-commit-extend	rewrites history
c F	add staged changes to existing commit	fix
	magit-commit-instant-fixup	rewrites history

In the commit buffer:

Key	Description	Mnemonic
C-c C-c	confirm commit	confirm
C-c C-k	cancel commit	kill
M-p	show previous message	previous
M-n	show next message	next