name-rev: don't _peek() in create_or_update_name()

name-rev: don't _peek() in create_or_update_name()

Look up the commit slab slot for the commit once using
commit_rev_name_at() and populate it in case it is empty, instead of
checking for emptiness in a separate step using commit_rev_name_peek()
via get_commit_rev_name().

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