grep: use return value of strbuf_detach()

grep: use return value of strbuf_detach()

Append the strbuf buffer only after detaching it.  There is no practical
difference here, as the strbuf is not empty and no strbuf_ function is
called between storing the pointer to the still attached buffer and
calling strbuf_detach(), so that pointer is valid, but make sure to
follow the standard sequence anyway for consistency.

Signed-off-by: René Scharfe <redacted>
Acked-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>