.vimrc: more default vimrc overrides

FreeBSD recently added a default vimrc.  Revert some of the
undesirable behaviour it introduces.
.vimrc: go to first line when editing a commit message

The "go to last cursor position" autocmd is generally useful but
annoying when editing git commit messages, which are always named
${GIT_DIR}/COMMIT_EDITMSG.  Always go to the first line when opening
a file thus named.