ls-tree: fix expansion of repeated %(path)

ls-tree: fix expansion of repeated %(path)

expand_show_tree() borrows the base strbuf given to us by read_tree() to
build the full path of the current entry when handling %(path).  Only
its indirect caller, show_tree_fmt(), removes the added entry name.
That works fine as long as %(path) is only included once in the format
string, but accumulates duplicates if it's repeated:

   $ git ls-tree --format='%(path) %(path) %(path)' HEAD M*
   Makefile MakefileMakefile MakefileMakefileMakefile

Reset the length after each use to get the same expansion every time;
here's the behavior with this patch:

   $ ./git ls-tree --format='%(path) %(path) %(path)' HEAD M*
   Makefile Makefile Makefile

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