modes/mirror: make Updated unchanged if no new commits fetched (#4341)

After sync mirror, get latest commit date and compare to current
repository updated time, only update it if the commit date is newer.

vendor/github.com/gogits/git-module/repo.go

@@ -278,3 +278,18 @@ func GetRepoSize(repoPath string) (*CountObject, error) {
 
 	return countObject, nil
 }
+
+// GetLatestCommitDate returns the date of latest commit of repository.
+// If branch is empty, it returns the latest commit across all branches.
+func GetLatestCommitDate(repoPath, branch string) (time.Time, error) {
+	cmd := NewCommand("for-each-ref", "--count=1", "--sort=-committerdate", "--format=%(committerdate:iso8601)")
+	if len(branch) > 0 {
+		cmd.AddArguments("refs/heads/" + branch)
+	}
+	stdout, err := cmd.RunInDir(repoPath)
+	if err != nil {
+		return time.Time{}, err
+	}
+
+	return time.Parse("2006-01-02 15:04:05 -0700", strings.TrimSpace(stdout))
+}