trace2: convert ctx.thread_name from strbuf to pointer
Convert the `tr2tls_thread_ctx.thread_name` field from a `strbuf`
to a "const char*" pointer.
The `thread_name` field is a constant string that is constructed when
the context is created. Using a (non-const) `strbuf` structure for it
caused some confusion in the past because it implied that someone
could rename a thread after it was created. That usage was not
intended. Change it to a const pointer to make the intent more clear.
Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>