Enhance /compare/ for tags and abbreviated commit shas

Gitea version (or commit ref): 55d9ddf

Description

This works:
/compare/d40cdb7ded29d1f406a2575ee03030b32f6d7114...2964016d5ae44fb587b96802f4e2bceadff6b2ee

This doesn't but probably should:
/compare/d40cdb7...2964016

This doesn't but probably should:
/compare/v1.1.1...v1.1.2

Basically anything that is commitish should compare with assumption 7 chars in the commit sha is enough to ensure uniqueness.

[saitho]

Supporting short hashes is related to src-d/go-git#602, since Gitea uses go-git to check if the reference exists.

[richmahn]

@saitho Do you know why though you can see a commit with a short commit hash? E.g. https://try.gitea.io/richmahn/test/commit/f713fac works for me

[saitho]

Hmm... haven't tried that yet. Maybe that's one doesn't rely on go-git? I'll look into that. :)

[richmahn]

I wonder if it has to do something in the likes of a git log | grep <short hash> or something to then find the long hash from the output?

[zeripath]

I just pushed a commit to the #6991 PR which adds the ability to use the commit SHA.