Fix#31271.
When gogit is enabled, `IsObjectExist` calls
`repo.gogitRepo.ResolveRevision`, which is not correct. It's for
checking references not objects, it could work with commit hash since
it's both a valid reference and a commit object, but it doesn't work
with blob objects.
So it causes #31271 because it reports that all blob objects do not
exist.
(cherry picked from commit f4d3120f9d1de6a260a5e625b3ffa6b35a069e9b)
Conflicts:
trivial resolution because go-git support was dropped https://codeberg.org/forgejo/forgejo/pulls/4941
83565de2c0