trace2: refactor Windows process ancestry trace2 event

trace2: refactor Windows process ancestry trace2 event

In 353d3d77f4 (trace2: collect Windows-specific process information,
2019-02-22) we added process ancestry information for Windows to TRACE2
via a data_json event. It was only later in 2f732bf15e (tr2: log parent
process name, 2021-07-21) that the specific cmd_ancestry event was
added to TRACE2.

In a future commit we will emit the ancestry information with the newer
cmd_ancestry TRACE2 event. Right now, we rework this implementation of
trace2_collect_process_info to separate the calculation of ancestors
from building and emiting the JSON array via a data_json event.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>