strbuf: pass correct alloc to strbuf_attach() in strbuf_reencode()

reencode_string_len() allocates len+1 bytes (including the NUL) and returns the string length in len. strbuf_reencode() was calling strbuf_attach(sb, out, len, len), so alloc was one byte too small. strbuf_attach() then calls strbuf_grow(sb, 0). With alloc < len+1, ALLOC_GROW always reallocates, so we reallocated immediately after attach even when the strbuf was not extended further. Pass len+1 as the alloc argument so the existing buffer is reused and the reallocation is avoided. Signed-off-by: Vaidas Pilkauskas <vaidas.pilkauskas@shopify.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>