upload: Split populate reviews into 2 phases

[jerry-skydio]

This sets the function up to allow arbitrary topic
order in the future.

The first pass handles finding the relative topic, but
does not assume it's already populated. The second pass
needs to assume that the relative is already populated
since it uses it, so it needs to be done in topological
order (which is a followup pr).

[jerry-skydio]

Reviews in this chain:
└#157 upload: Split populate reviews into 2 phases
　└#158 upload: Add topological_topics as a generator
　　└#155 upload: Allow arbitrary topic order
　　　├#153 upload: Support Update-Pr-Body as a commit tag
　　　│└#167 upload: Add "branch-format" config option and commit tag
　　　└#165 restack: Be topological and update docs

[jerry-skydio]

#	head	base	diff	date	summary
0	edb2370d	df295937	diff	Mar 12 14:07 PM	1 file changed, 9 insertions(+), 6 deletions(-)
1	0a5408f3	df295937	diff	Mar 12 15:16 PM	1 file changed, 25 insertions(+), 22 deletions(-)
2	ad555d4a	df295937	diff	Mar 12 15:20 PM	0 files changed
3	8b3358d6	df295937	diff	Mar 12 15:25 PM	1 file changed, 7 insertions(+), 7 deletions(-)
4	fc580cd3	df295937	diff	Mar 13 13:40 PM	2 files changed, 3 insertions(+), 4 deletions(-)
5	4231fd44	139ad626	rebase	Mar 13 13:46 PM	0 files changed