environment: do not use strbuf_split*()

environment: do not use strbuf_split*()

environment.c:get_git_namespace() learns the raw namespace from an
environment variable, splits it at "/", and appends them after
"refs/namespaces/"; the reason why it splits first is so that an
empty string resulting from double slashes can be omitted.

The split pieces do not need to be edited in any way, so an array of
strbufs is a wrong data structure to use.  Instead split into a
string list and use the pieces from there.

Signed-off-by: Junio C Hamano <gitster@pobox.com>