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

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>