Is there any way I can do

git add -A
git commit -m "commit message"

in one command?

I seem to be doing those two commands a lot, and if Git had an option like git commit -Am "commit message", it would make life that much more convenient.

git commit has the -a modifier, but it doesn't quite do the same as doing git add -A before committing. git add -A adds newly created files, but git commit -am does not. What does?

You can use git aliases, e.g.

git config --global alias.add-commit '!git add -A && git commit'

and use it with

git add-commit -m 'My commit message'

EDIT: Reverted back to ticks ('), as otherwise it will fail for shell expansion on Linux. On Windows, one should use double-quotes (") instead (pointed out in the comments, did not verify).