strbuf: use st_add3() in strbuf_grow()

strbuf: use st_add3() in strbuf_grow()

Simplify the code by calling st_add3() to do overflow checks instead of
open-coding it.  This changes the error message to include the offending
summands, which can be helpful when tracking down the cause.

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