trace2: add dots directly to strbuf in perf_fmt_prepare()

trace2: add dots directly to strbuf in perf_fmt_prepare()

The initialization function of the Trace2 performance format target sets
aside a stash of dots for indenting output.  Get rid of it and use
strbuf_addchars() to provide dots on demand instead.  This shortens the
code, gets rid of a small heap allocation and is a bit more efficient.

Signed-off-by: René Scharfe <redacted>
Acked-by: Jeff King <redacted>
Acked-by: Jeff Hostetler <redacted>
Signed-off-by: Junio C Hamano <redacted>