upload: Add topological_topics as a generator

[jerry-skydio]

Add recursive helper generator functions to iterate
topologically through topics, and use it in some key
places. Also detect cycles in the topic graph.

[jerry-skydio]

Reviews in this chain:
└#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	91d3c784	edb2370d	diff	Mar 12 14:46 PM	1 file changed, 29 insertions(+), 2 deletions(-)
1	e4431364	edb2370d	diff	Mar 12 14:48 PM	1 file changed, 7 insertions(+), 9 deletions(-)
2	0a455aa1	edb2370d	diff	Mar 12 15:04 PM	1 file changed, 10 insertions(+), 4 deletions(-)
3	b056a294	edb2370d	diff	Mar 12 15:07 PM	1 file changed, 4 insertions(+), 5 deletions(-)
4	09150f0d	0a5408f3	diff	Mar 12 15:16 PM	1 file changed, 25 insertions(+), 22 deletions(-)
5	04de160d	ad555d4a	diff	Mar 12 15:20 PM	0 files changed
6	95840c7a	8b3358d6	rebase	Mar 12 15:25 PM	0 files changed
7	30b14996	8b3358d6	diff	Mar 12 15:27 PM	1 file changed, 1 insertion(+), 2 deletions(-)
8	250ab201	8b3358d6	diff	Mar 13 10:58 AM	1 file changed, 12 insertions(+), 13 deletions(-)
9	d3a97751	8b3358d6	diff	Mar 13 11:59 AM	1 file changed, 1 insertion(+), 2 deletions(-)
10	6c701b7b	8b3358d6	diff	Mar 13 12:03 PM	1 file changed, 1 insertion(+), 2 deletions(-)
11	5d08aabe	fc580cd3	rebase	Mar 13 13:40 PM	0 files changed
12	eea47247	4231fd44	rebase	Mar 13 13:46 PM	0 files changed
13	01112953	2dfbe6ea	diff	Mar 13 13:50 PM	0 files changed

[aaron-skydio]

Cool