Improve opengraph previews (#26851)

Add more useful Open Graph metadata for commit and file URLs:

- Set `og:title` to the page title, which is a concise summary in both
cases (`<commit message> · <commit hash>` and `<filename> at <branch>`,
respectively)
- Set `og:description` to the commit message body, if available
- Set `og:url` to the relevant URLs instead of the repo URL

Also move the relevant meta tags into a separate template as they now
take up the majority of the base head template.

templates/base/head.tmpl

@@ -25,42 +25,7 @@
 			.ui.secondary.menu .dropdown.item > .menu { margin-top: 0; }
 		</style>
 	</noscript>
-{{if .PageIsUserProfile}}
-	<meta property="og:title" content="{{.ContextUser.DisplayName}}">
-	<meta property="og:type" content="profile">
-	<meta property="og:image" content="{{.ContextUser.AvatarLink ctx}}">
-	<meta property="og:url" content="{{.ContextUser.HTMLURL}}">
-	{{if .ContextUser.Description}}
-		<meta property="og:description" content="{{.ContextUser.Description}}">
-	{{end}}
-{{else if .Repository}}
-	{{if .Issue}}
-		<meta property="og:title" content="{{.Issue.Title}}">
-		<meta property="og:url" content="{{.Issue.HTMLURL}}">
-		{{if .Issue.Content}}
-			<meta property="og:description" content="{{.Issue.Content}}">
-		{{end}}
-	{{else}}
-		<meta property="og:title" content="{{.Repository.Name}}">
-		<meta property="og:url" content="{{.Repository.HTMLURL}}">
-		{{if .Repository.Description}}
-			<meta property="og:description" content="{{.Repository.Description}}">
-		{{end}}
-	{{end}}
-	<meta property="og:type" content="object">
-	{{if (.Repository.AvatarLink ctx)}}
-		<meta property="og:image" content="{{.Repository.AvatarLink ctx}}">
-	{{else}}
-		<meta property="og:image" content="{{.Repository.Owner.AvatarLink ctx}}">
-	{{end}}
-{{else}}
-	<meta property="og:title" content="{{AppName}}">
-	<meta property="og:type" content="website">
-	<meta property="og:image" content="{{AssetUrlPrefix}}/img/logo.png">
-	<meta property="og:url" content="{{AppUrl}}">
-	<meta property="og:description" content="{{MetaDescription}}">
-{{end}}
-	<meta property="og:site_name" content="{{AppName}}">
+	{{template "base/head_opengraph" .}}
 	{{template "base/head_style" .}}
 	{{template "custom/header" .}}
 </head>

templates/base/head_opengraph.tmpl

@@ -0,0 +1,42 @@
+{{if .PageIsUserProfile}}
+	<meta property="og:title" content="{{.ContextUser.DisplayName}}">
+	<meta property="og:type" content="profile">
+	<meta property="og:image" content="{{.ContextUser.AvatarLink ctx}}">
+	<meta property="og:url" content="{{.ContextUser.HTMLURL}}">
+	{{if .ContextUser.Description}}
+		<meta property="og:description" content="{{.ContextUser.Description}}">
+	{{end}}
+{{else if .Repository}}
+	{{if .Issue}}
+		<meta property="og:title" content="{{.Issue.Title}}">
+		<meta property="og:url" content="{{.Issue.HTMLURL}}">
+		{{if .Issue.Content}}
+			<meta property="og:description" content="{{.Issue.Content}}">
+		{{end}}
+	{{else if or .PageIsDiff .IsViewFile}}
+		<meta property="og:title" content="{{.Title}}">
+		<meta property="og:url" content="{{AppUrl}}{{.Link}}">
+		{{if and .PageIsDiff (IsMultilineCommitMessage .Commit.Message)}}
+			<meta property="og:description" content="{{RenderCommitBody $.Context .Commit.Message $.RepoLink $.Repository.ComposeMetas}}">
+		{{end}}
+	{{else}}
+		<meta property="og:title" content="{{.Repository.Name}}">
+		<meta property="og:url" content="{{.Repository.HTMLURL}}">
+		{{if .Repository.Description}}
+			<meta property="og:description" content="{{.Repository.Description}}">
+		{{end}}
+	{{end}}
+	<meta property="og:type" content="object">
+	{{if (.Repository.AvatarLink ctx)}}
+		<meta property="og:image" content="{{.Repository.AvatarLink ctx}}">
+	{{else}}
+		<meta property="og:image" content="{{.Repository.Owner.AvatarLink ctx}}">
+	{{end}}
+{{else}}
+	<meta property="og:title" content="{{AppName}}">
+	<meta property="og:type" content="website">
+	<meta property="og:image" content="{{AssetUrlPrefix}}/img/logo.png">
+	<meta property="og:url" content="{{AppUrl}}">
+	<meta property="og:description" content="{{MetaDescription}}">
+{{end}}
+<meta property="og:site_name" content="{{AppName}}">