Change gitcommit syntax match names.

vim/ftplugin/gitcommit.vim
	-Change syntax-match names from camel-case to snake-case, for
	consistency with my other syntax files.
	-Define a match for abbreviated commit hashes.
Tweak cinit makefile debug-target flag.

res/cinit/make.tmp
	-The `-Og` flag is superior to `-O0`, because it implements as
	many optimizations as possible while explicitly preserving
	debugging symbols, neither of which the latter flag does.