fix: release page for empty or non-existing target (#24470)

Fixes #24145

To solve the bug, I added a "computed" `TargetBehind` field to the
`Release` model, which indicates the target branch of a release.
This is particularly useful if the target branch was deleted in the
meantime (or is empty).

I also did a micro-optimization in `calReleaseNumCommitsBehind`. Instead
of checking that a branch exists and then call `GetBranchCommit`, I
immediately call `GetBranchCommit` and handle the `git.ErrNotExist`
error.

This optimization is covered by the added unit test.

models/fixtures/release.yml

@@ -108,3 +108,31 @@
   is_prerelease: false
   is_tag: false
   created_unix: 946684803
+
+- id: 9
+  repo_id: 57
+  publisher_id: 2
+  tag_name: "non-existing-target-branch"
+  lower_tag_name: "non-existing-target-branch"
+  target: "non-existing"
+  title: "non-existing-target-branch"
+  sha1: "cef06e48f2642cd0dc9597b4bea09f4b3f74aad6"
+  num_commits: 5
+  is_draft: false
+  is_prerelease: false
+  is_tag: false
+  created_unix: 946684803
+
+- id: 10
+  repo_id: 57
+  publisher_id: 2
+  tag_name: "empty-target-branch"
+  lower_tag_name: "empty-target-branch"
+  target: ""
+  title: "empty-target-branch"
+  sha1: "cef06e48f2642cd0dc9597b4bea09f4b3f74aad6"
+  num_commits: 5
+  is_draft: false
+  is_prerelease: false
+  is_tag: false
+  created_unix: 946684803

models/repo/release.go

@@ -72,6 +72,7 @@ type Release struct {
 	OriginalAuthorID int64 `xorm:"index"`
 	LowerTagName     string
 	Target           string
+	TargetBehind     string `xorm:"-"` // to handle non-existing or empty target
 	Title            string
 	Sha1             string `xorm:"VARCHAR(40)"`
 	NumCommits       int64