backporting: Suggest only one related commit for a backport

[joestringer]

Recently, the check-stable script has suggested every single possible
match for commits where the name does not uniquely identify the commit.
This can be a bit confusing to backporters since it looks like there are
many commits to backport as part of this PR, but the second and later
ones are not necessary to backport.

 * PR: 16589 -- vagrant: Bump all Vagrant box versions (@pchaigno) -- https://github.com/cilium/cilium/pull/16589
   Merge with 1 commit(s) merged at: Tue, 22 Jun 2021 12:36:17 -0700!       
     Branch:     master (!)                          refs/pull/16589/head   
                 ----------                          -------------------    
     v (start)                                                              
     |  edf76fb1ef6b58d5ef90b439d54134f314ed086e                            
5bef5d77137a9ecc5d3f2b72149307ffdd52cd42                                    
4dc60e6faf654d7424ee959867a774205b3fed13                                    
816b3231cdbc39f4bcdd3e6f5b40a056459a478c                                    
51826b31087496d108044f3bffbf304580fffb4a                                    
df8238d451d755d5be75e202be89b4f88067c77b                                    
a4e7bc6c1f0e96078793458b6719b9a3999b89db via fb723f8133c40faa068a5a401f594622668b2753 ("vagrant: Bump all Vagrant box versions")
     v (end)                                                                

Probably within the last year of commits, we should be able to correlate
the exact commit that needs backporting, so iterate through those to
find the exact commit. If none of those are the correct commit, fail out
and push back to the backporter to figure out.

This allows us to now accurately pick the correct commit in most cases:

 * PR: 16589 -- vagrant: Bump all Vagrant box versions (@pchaigno) -- https://github.com/cilium/cilium/pull/16589
   Merge with 1 commit(s) merged at: Tue, 22 Jun 2021 12:36:17 -0700!       
     Branch:     master (!)                          refs/pull/16589/head   
                 ----------                          -------------------    
     v (start)                                                              
     |  edf76fb1ef6b58d5ef90b439d54134f314ed086e via fb723f8133c40faa068a5a401f594622668b2753 ("vagrant: Bump all Vagrant box versions")
     v (end)                                                                

Manually tested by substituting a known commit into 'related_commits',
and by checking the current v1.8 backports which includes an ambiguous
commit due to a revert+reapply in the master branch.

Fixes: #16572

[christarazi]

🚀

[pchaigno]

I think this is creating ambiguity where there isn't:

Found the following ambiguous commits:
edf76fb paul@cilium.io 2021-06-18 vagrant: Bump all Vagrant box versions
5bef5d7 paul@cilium.io 2021-04-21 vagrant: Bump all Vagrant box versions
4dc60e6 paul@cilium.io 2021-04-19 vagrant: Bump all Vagrant box versions
816b323 paul@cilium.io 2020-11-25 vagrant: Bump all Vagrant box versions
51826b3 paul@cilium.io 2020-11-13 vagrant: Bump all Vagrant box versions
df8238d paul@cilium.io 2020-09-30 vagrant: Bump all Vagrant box versions
a4e7bc6 paul@cilium.io 2020-07-23 vagrant: Bump all Vagrant box versions

How are those commits ambiguous? They don't have the same author date at all. If they don't have the same author dates, they are not the same, regardless of commit titles.

Printing them all is only likely to confuse backporters IMO. What are they suppose to do with that information? Check every commit? If they should just check the dates match, why not automate that part?

[joestringer]

👍 good feedback @pchaigno , and thanks for being patient with me in Slack 😅

I plan to respin this some time this week.

[pchaigno]

LGTM, with one suggestion below.