tree.h API: simplify read_tree_recursive() signature

tree.h API: simplify read_tree_recursive() signature

Simplify the signature of read_tree_recursive() to omit the "base",
"baselen" and "stage" arguments. No callers of it use these parameters
for anything anymore.

The last function to call read_tree_recursive() with a non-"" path was
read_tree_recursive() itself, but that was changed in
ffd31f661d5 (Reimplement read_tree_recursive() using
tree_entry_interesting(), 2011-03-25).

The last user of the "stage" parameter went away in the last commit,
and even that use was mere boilerplate.

So let's remove those and rename the read_tree_recursive() function to
just read_tree(). We had another read_tree() function that I've
refactored away in preceding commits, since all in-tree users read
trees recursively with a callback we can change the name to signify
that this is the norm.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>