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/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