ls-tree: rename "retval" to "recurse" in "show_tree()"

ls-tree: rename "retval" to "recurse" in "show_tree()"

The variable which "show_tree()" return is named "retval", a name that's
a little hard to understand. The commit rename "retval" to "recurse"
which is a more meaningful name than before in the context. We do not
need to take a look at "read_tree_at()" in "tree.c" to make sure what
does "retval" mean.

Signed-off-by: Teng Long <dyroneteng@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>