trace2: rename the thread_name argument to trace2_thread_start

trace2: rename the thread_name argument to trace2_thread_start

Rename the `thread_name` argument in `tr2tls_create_self()` and
`trace2_thread_start()` to be `thread_base_name` to make it clearer
that the passed argument is a component used in the construction of
the actual `struct tr2tls_thread_ctx.thread_name` variable.

The base name will be used along with the thread id to create a
unique thread name.

This commit does not change how the `thread_name` field is
allocated or stored within the `tr2tls_thread_ctx` structure.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>