cache-tree: fix strbuf growth in prime_cache_tree_rec()

cache-tree: fix strbuf growth in prime_cache_tree_rec()

Use size_t to store the original length of the strbuf tree_len, as
that's the correct type.

Don't double the allocated size of the strbuf when adding a subdirectory
name.  And the chance of the trailing slash fitting in the slack left by
strbuf_add() is very high, so stop pre-growing the strbuf at all.

Suggested-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>