rm: reuse strbuf for all remove_dir_recursively() calls, again

rm: reuse strbuf for all remove_dir_recursively() calls, again

Don't throw the memory allocated for remove_dir_recursively() away after
a single call, use it for the other entries as well instead.

This change was done before in deb8e15a (rm: reuse strbuf for all
remove_dir_recursively() calls), but was reverted as a side-effect of
55856a35 (rm: absorb a submodules git dir before deletion). Reinstate
the optimization.

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