git-mktree: reverse of git-ls-tree.

git-mktree: reverse of git-ls-tree.

This reads data in the format a (non recursive) ls-tree outputs
and writes a tree object to the object database.  The created
tree object name is output to the standard output.

For convenience, the input data does not need to be sorted; the
command sorts the input lines internally.

By request from Tommi Virtanen.

Signed-off-by: Junio C Hamano <junkio@cox.net>