How to track but not stage and how to unstage but not untrack?

Question

I have two concise questions:

• How I can track files but without staging them ?
• How I can unstage files for commit without untracking them ?

Answer 1

Update (May 2015)

I tried to use git add -N but it tracks the file and add it for commit:

That is no longer the case with the upcoming Git 2.5 (Q2 2015).
See "File doesn′t get into the commit after using git add -N"

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Original answer (March 2013)

How I can unstage files for commit without untracking them ?

this is the official way:

git reset HEAD fileA

But since it is a new file, you would untrack it as well (remove it from the index, without any previous commit referencing it).

Starting tracking a file means having it in a index (stage) or a commit.

I would recommend making a branch for those files, in order to add them/commit them there.
See "What is Tracked files and Untracked files in the context of GIT?"

• Tracked files are files that were in the last snapshot; they can be unmodified, modified, or staged.
• Untracked files are everything else — any files in your working directory that were not in your last snapshot and are not in your staging area (index)

That means that, for a new file, unstaged it means untrack it.

^{(Source: Pro Git Book, 2.2 Git Basics - Recording Changes to the Repository)}
(Thank you, louisfischer, for the update/fix in the comments)

See also "git - how to tell if a file is git tracked (by shell exit code)?".