strbuf: use _rtrim and _ltrim in strbuf_trim

strbuf_trim() strips whitespace from the end, then the beginning of
a strbuf.  Those operations are duplicated in strbuf_rtrim() and
strbuf_ltrim().

Replace strbuf_trim() implementation with calls to strbuf_rtrim(),
then strbuf_ltrim().

Signed-off-by: Brian Gesiak <modocache@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
api-strbuf.txt: add docs for _trim and _ltrim

API documentation for strbuf does not document strbuf_trim() or
strbuf_ltrim(). Add documentation for these two functions.

Signed-off-by: Brian Gesiak <modocache@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>