git write-tree [--missing-ok] [--prefix=<prefix>/]
Creates a tree object using the current index. The name of the new tree object is printed to standard output.
The index must be in a fully merged state.
Conceptually, git write-tree sync()s the current index contents into a set of tree files. In order to have that match what is actually in your directory right now, you need to have done a git update-index phase before you did the git write-tree.
Part of the git(1) suite