read-tree -m: make error message for merging 0 trees less smart aleck

read-tree -m: make error message for merging 0 trees less smart aleck

"git read-tree -m" requires a tree argument to name the tree to be
merged in.  Git uses a cutesy error message to say so and why:

    $ git read-tree -m
    warning: read-tree: emptying the index with no arguments is
    deprecated; use --empty
    fatal: just how do you expect me to merge 0 trees?
    $ git read-tree -m --empty
    fatal: just how do you expect me to merge 0 trees?

When lucky, that could produce an ah-hah moment for the user, but it's
more likely to irritate and distract them.

Instead, tell the user plainly that the tree argument is
required. Also document this requirement in the git-read-tree(1)
manpage where there is room to explain it in a more straightforward way.

Signed-off-by: Jean-Noel Avila <jn.avila@free.fr>
Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>