Fix IsObjectExist with gogit (#31790)

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.

modules/git/repo_branch_nogogit.go

@@ -16,7 +16,7 @@ import (
 	"code.gitea.io/gitea/modules/log"
 )
 
-// IsObjectExist returns true if given reference exists in the repository.
+// IsObjectExist returns true if the given object exists in the repository.
 func (repo *Repository) IsObjectExist(name string) bool {
 	if name == "" {
 		return false