upload: Allow arbitrary topic order

[jerry-skydio]

Now that we're iterating over topics topologically in the important places, we can remove the check that requires topics to be in consecutive order.

This means revup is able to pull together relative chains in any arbitrary order, as long as that order is valid with no cycles.

Fixes: #156

[jerry-skydio]

Reviews in this chain: └https://github.com/Skydio/revup/pull/155 upload: Allow arbitrary topic order 　├https://github.com/Skydio/revup/pull/153 upload: Support Update-Pr-Body as a commit tag 　│└https://github.com/Skydio/revup/pull/167 upload: Add "branch-format" config option and commit tag 　└https://github.com/Skydio/revup/pull/165 restack: Be topological and update docs

[jerry-skydio]

#	head	base	diff	date	summary
0	f2506f16	df295937	diff	Mar 12 13:31 PM	1 file changed, 14 insertions(+), 19 deletions(-)
1	2c810f31	91d3c784	diff	Mar 12 14:46 PM	1 file changed, 38 insertions(+), 10 deletions(-)
2	b8f56cad	e4431364	diff	Mar 12 14:48 PM	1 file changed, 6 insertions(+), 8 deletions(-)
3	06778e61	e4431364	diff	Mar 12 14:49 PM	0 files changed
4	57328501	0a455aa1	rebase	Mar 12 15:04 PM	0 files changed
5	cdfceb11	b056a294	rebase	Mar 12 15:07 PM	0 files changed
6	fd95dd87	b056a294	diff	Mar 12 15:08 PM	1 file changed, 1 insertion(+), 3 deletions(-)
7	bc4eeaaf	09150f0d	rebase	Mar 12 15:16 PM	0 files changed
8	58d26e76	04de160d	diff	Mar 12 15:20 PM	0 files changed
9	a2975ed8	95840c7a	rebase	Mar 12 15:25 PM	0 files changed
10	ed750a85	30b14996	rebase	Mar 12 15:27 PM	0 files changed
11	b48ebf47	250ab201	rebase	Mar 13 10:58 AM	0 files changed
12	2d8cc8e9	d3a97751	rebase	Mar 13 11:59 AM	0 files changed
13	65cc1b54	6c701b7b	rebase	Mar 13 12:03 PM	0 files changed
14	f3f012a2	5d08aabe	rebase	Mar 13 13:40 PM	0 files changed
15	1d3dfa14	eea47247	rebase	Mar 13 13:46 PM	0 files changed
16	79d040b3	01112953	rebase	Mar 13 13:50 PM	0 files changed
17	9a5a6e35	bb5f5040	diff	Mar 13 14:02 PM	0 files changed