ls-tree: fix --no-full-name

ls-tree: fix --no-full-name

Since 61fdbcf98b (ls-tree: migrate to parse-options, 2009-11-13) git
ls-tree has accepted the option --no-full-name, but it does the same
as --full-name, contrary to convention.  That's because it's defined
using OPT_SET_INT with a value of 0, where the negative variant sets
0 as well.

Turn --no-full-name into the opposite of --full-name by using OPT_BOOL
instead and storing the option's status directly in a variable named
"full_name" instead of in negated form in "chomp_prefix".

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>