strbuf.c: add `strbuf_insertf()` and `strbuf_vinsertf()`
Implement `strbuf_insertf()` and `strbuf_vinsertf()` to
insert data using a printf format string.
Original-idea-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Paul-Sebastian Ungureanu <ungureanupaulsebastian@gmail.com> Helped-by: Johannes Sixt <j6t@kdbg.org> Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>