strbuf_branchname: drop return value

strbuf_branchname: drop return value

The return value from strbuf_branchname() is confusing and
useless: it's 0 if the whole name was consumed by an @-mark,
but otherwise is the length of the original name we fed.

No callers actually look at the return value, so let's just
get rid of it.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>