Fix bug when visiting comparation page (#34334) (#34365)

Backport #34334 by @lunny

The `ci.HeadGitRepo` was opened and closed in the function
`ParseCompareInfo` but reused in the function `PrepareCompareDiff`.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

routers/web/repo/compare.go

@@ -401,12 +401,11 @@ func ParseCompareInfo(ctx *context.Context) *common.CompareInfo {
 		ci.HeadRepo = ctx.Repo.Repository
 		ci.HeadGitRepo = ctx.Repo.GitRepo
 	} else if has {
-		ci.HeadGitRepo, err = gitrepo.OpenRepository(ctx, ci.HeadRepo)
+		ci.HeadGitRepo, err = gitrepo.RepositoryFromRequestContextOrOpen(ctx, ci.HeadRepo)
 		if err != nil {
-			ctx.ServerError("OpenRepository", err)
+			ctx.ServerError("RepositoryFromRequestContextOrOpen", err)
 			return nil
 		}
-		defer ci.HeadGitRepo.Close()
 	} else {
 		ctx.NotFound(nil)
 		return nil
@@ -721,11 +720,6 @@ func getBranchesAndTagsForRepo(ctx gocontext.Context, repo *repo_model.Repositor
 // CompareDiff show different from one commit to another commit
 func CompareDiff(ctx *context.Context) {
 	ci := ParseCompareInfo(ctx)
-	defer func() {
-		if ci != nil && ci.HeadGitRepo != nil {
-			ci.HeadGitRepo.Close()
-		}
-	}()
 	if ctx.Written() {
 		return
 	}

routers/web/repo/pull.go

@@ -1290,11 +1290,6 @@ func CompareAndPullRequestPost(ctx *context.Context) {
 	)
 
 	ci := ParseCompareInfo(ctx)
-	defer func() {
-		if ci != nil && ci.HeadGitRepo != nil {
-			ci.HeadGitRepo.Close()
-		}
-	}()
 	if ctx.Written() {
 		return
 	}