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

---------

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

routers/web/repo/wiki.go

@@ -334,9 +334,6 @@ func renderRevisionPage(ctx *context.Context) (*git.Repository, *git.TreeEntry)
 	ctx.Data["Title"] = displayName
 	ctx.Data["title"] = displayName
 
-	ctx.Data["Username"] = ctx.Repo.Owner.Name
-	ctx.Data["Reponame"] = ctx.Repo.Repository.Name
-
 	// lookup filename in wiki - get page content, gitTree entry , real filename
 	_, entry, pageFilename, noEntry := wikiContentsByName(ctx, commit, pageName)
 	if noEntry {