Change the name of real_path_internal to strbuf_realpath. In addition
push the static strbuf up to its callers and instead take as a
parameter a pointer to a strbuf to use for the final result.
This change makes strbuf_realpath reentrant.
Signed-off-by: Brandon Williams <bmwill@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>