trace: use strbuf for quote_crnl output

trace: use strbuf for quote_crnl output

When we output GIT_TRACE_SETUP paths, we quote any
meta-characters. But our buffer to hold the result is only
PATH_MAX bytes, and we could double the size of the input
path (if every character needs quoting). We could use a
2*PATH_MAX buffer, if we assume the input will never be more
than PATH_MAX. But it's easier still to just switch to a
strbuf and not worry about whether the input can exceed
PATH_MAX or not.

The original copied the "p2" pointer to "p1", advancing
both. Since this gets rid of "p1", let's also drop "p2",
whose name is now confusing. We can just advance the
original "path" pointer.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>