Refactor compare diff/pull page (1) (#37481)

1. Rename CompareInfo.MergeBase to CompareBase, it is not merge base
2. Remove unused template variables `ctx.Data["Username"]` and
`ctx.Data["Reponame"]`
3. Decouple some template variable accesses, use typed struct

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Co-authored-by: Nicolas <bircni@icloud.com>

routers/web/repo/pull_review.go

@@ -169,7 +169,7 @@ func UpdateResolveConversation(ctx *context.Context) {
 func renderConversation(ctx *context.Context, comment *issues_model.Comment, origin string) {
 	ctx.Data["PageIsPullFiles"] = origin == "diff"
 
-	showOutdatedComments := origin == "timeline" || ctx.Data["ShowOutdatedComments"].(bool)
+	showOutdatedComments := origin == "timeline" || GetShowOutdatedComments(ctx)
 	comments, err := issues_model.FetchCodeCommentsByLine(ctx, comment.Issue, ctx.Doer, comment.TreePath, comment.Line, showOutdatedComments)
 	if err != nil {
 		ctx.ServerError("FetchCodeCommentsByLine", err)