Add public API endpoint for finding repos by commit (including forks)

[SoniEx2]

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Description

I'd like a public API endpoint that lists the repos/branches that contain a given commit.

Screenshots

[SoniEx2]

More context: I am currently working on this thing https://ganarchy.autistic.space/

I'd like it to be more efficient and perhaps integrate it into gitea at some later point. Mainly just more efficient, i.e. it should be able to run on shitbox VPSes.