sha1_name: simplify strbuf handling in interpret_nth_prior_checkout()

sha1_name: simplify strbuf handling in interpret_nth_prior_checkout()

Pass the target strbuf to the callback function grab_nth_branch_switch()
by reference so that it can add the result string directly instead of
having it put the string into a temporary strbuf first.  This gets rid
of an extra allocation and a string copy.

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