upload: Query head oid of a PR via commits

[jerry-skydio]

The headRefOid field of a PR actually contains the oid of
whatever the tot of that branch name is, which may not be the
commit that is actually associated with the pr if it has been merged.
To get that correct, we have to query the actual commit list.

This handles a corner case in reordering PRs with the repo's merge
setting as rebase. Somehow this causes one of the reordered PRs to
get merged with 0 commmits, which later causes us to not re-make prs.

With this change, we'll always succeed in remaking prs if they are
erroneously closed or merged on a reorder. Avoiding having them get
closed is a follow-up change.

Topic: query1
Reviewers: aaron, brian-k

[jerry-skydio]

Reviews in this chain:
└#206 upload: Query head oid of a PR via commits

[jerry-skydio]

#	head	base	diff	date	summary
0	f1278934	c7b92e83	diff	Dec 30 0:28 AM	2 files changed, 37 insertions(+), 14 deletions(-)
1	935eac18	159f964c	diff	Dec 30 15:10 PM	1 file changed, 1 insertion(+)
2	e3cf33ed	159f964c	diff	Dec 30 15:12 PM	0 files changed
3	6ceee4e5	159f964c	diff	Dec 30 15:40 PM	1 file changed, 1 deletion(-)
4	c5af2774	0d7e5d3a	rebase	Jan 2 13:54 PM	0 files changed
5	65c5638d	0d7e5d3a	diff	Jan 2 14:05 PM	1 file changed, 1 insertion(+), 1 deletion(-)