On Thu, 30 Nov 2006, Linus Torvalds wrote:

The default is: commit everything that you ask for to be committed.

If you haven't marked anything to be committed (which you can do with "git 
add" too, or with simply being in the middle of a merge, or by having done 
something like "git pull -n" or similar that does everything _but_ 
commit), then git commit will say "nothing to do".

Might it be a good idea to have "git-add" do the same as 
"git-update-index" on already tracked files?  That could be easily 
taught as "you must explicitly _add_ files to your next commit" and 
whether the file is already tracked or not wouldn't matter.  This would 
help newbies actually getting used the index without mentioning the 
dreaded word "index" at all.

Right now git-add on an already tracked file does nothing, not even a 
message to say it did nothing.