NERDTree is launched when there are no args.

My first vimscript!

This meets my expectations well, as going to a folder and using vim
usually means I want nerdtree. If I open a specific file, I usually
do not want nerdtree. Git commits are files at .git/COMMIT_EDITMSG.

The vimscript is obviously incredibly simple. It was harder to find
the right commands to handle the argument list than anything else.

argc()
    Count of arguments
argv()
    Array of arguments

You can index the array, so I added .git/COMMIT_EDITMSG here using:

<C-R>=argc()[0]

(from insert mode).