strbuf_add_wrapped_text(): factor out strbuf_add_indented_text()

strbuf_add_wrapped_text(): factor out strbuf_add_indented_text()

Add a new helper function, strbuf_add_indented_text(), to indent text
without a width limit, and call it from strbuf_add_wrapped_text().  It
respects both indent (applied to the first line) and indent2 (applied to
the rest of the lines); indent2 was ignored by the indent-only path of
strbuf_add_wrapped_text() before the patch.

Two simple test cases are added, one exercising strbuf_add_wrapped_text()
and the other strbuf_add_indented_text().

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>