fix: git cache (#38763)

1. always use "last commit cache"
2. correctly build the cache key for any input (SafeCacheKey)
3. fix the git note "last commit cache FIXME" and avoid OOM

modules/git/commit_count_cache.go

@@ -0,0 +1,27 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package git
+
+import (
+	"context"
+
+	"gitea.dev/modules/cache"
+)
+
+func makeCommitsCountCacheKey(repo RepositoryFacade, ref RefName) string {
+	return cache.SafeCacheKey("git-commits-count:"+repo.GitRepoManagedID(), ref.String())
+}
+
+func RemoveCommitsCountCache(repo RepositoryFacade, ref RefName) {
+	cache.Remove(makeCommitsCountCacheKey(repo, ref))
+}
+
+func GetCommitsCountCache(ctx context.Context, repo RepositoryFacade, ref RefName, commit *Commit) (int64, error) {
+	if commit == nil {
+		return 0, nil
+	}
+	return cache.GetInt64(makeCommitsCountCacheKey(repo, ref), func() (int64, error) {
+		return CommitsCountOfCommit(ctx, repo, commit.ID.String())
+	})
+}