Fix tags sort by creation time (descending) on branch/tag dropdowns (#23491)

This PR fixes the tags sort issue mentioned in #23432
The tags on dropdown shoud be sorted in descending order of time but are
not. Because when getting tags, it execeutes `git tag sort
--sort=-taggerdate`. Git supports two types of tags: lightweight and
annotated, and `git tag sort --sort=-taggerdate` dosen't work with
lightweight tags, which will not give correct result. This PR add
`GetTagNamesByRepoID ` to get tags from the database so the tags are
sorted.

Also adapt this change to the droplist when comparing branches. 

Dropdown places:

<img width="369" alt="截屏2023-03-15 14 25 39"
src="https://user-images.githubusercontent.com/17645053/225224506-65a72e50-4c11-41d7-8187-a7e9c7dab2cb.png">

<img width="675" alt="截屏2023-03-15 14 25 27"
src="https://user-images.githubusercontent.com/17645053/225224526-65ce8008-340c-43f6-aa65-b6bd9e1a1bf1.png">

models/repo/release.go

@@ -253,6 +253,28 @@ func GetReleasesByRepoID(ctx context.Context, repoID int64, opts FindReleasesOpt
 	return rels, sess.Find(&rels)
 }
 
+// GetTagNamesByRepoID returns a list of release tag names of repository.
+func GetTagNamesByRepoID(ctx context.Context, repoID int64) ([]string, error) {
+	listOptions := db.ListOptions{
+		ListAll: true,
+	}
+	opts := FindReleasesOptions{
+		ListOptions:   listOptions,
+		IncludeDrafts: true,
+		IncludeTags:   true,
+		HasSha1:       util.OptionalBoolTrue,
+	}
+
+	tags := make([]string, 0)
+	sess := db.GetEngine(ctx).
+		Table("release").
+		Desc("created_unix", "id").
+		Where(opts.toConds(repoID)).
+		Cols("tag_name")
+
+	return tags, sess.Find(&tags)
+}
+
 // CountReleasesByRepoID returns a number of releases matching FindReleaseOptions and RepoID.
 func CountReleasesByRepoID(repoID int64, opts FindReleasesOptions) (int64, error) {
 	return db.GetEngine(db.DefaultContext).Where(opts.toConds(repoID)).Count(new(Release))