Fix IsObjectExist with gogit (#31790) (tests only)

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

modules/markup/html.go

@@ -1197,7 +1197,7 @@ func hashCurrentPatternProcessor(ctx *RenderContext, node *html.Node) {
 				})
 			}
 
-			exist = ctx.GitRepo.IsObjectExist(hash)
+			exist = ctx.GitRepo.IsReferenceExist(hash)
 			ctx.ShaExistCache[hash] = exist
 		}