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/api/v1/repo/pull.go

@@ -521,7 +521,7 @@ func CreatePullRequest(ctx *context.APIContext) {
 		BaseBranch: compareResult.BaseRef.ShortName(),
 		HeadRepo:   compareResult.HeadRepo,
 		BaseRepo:   repo,
-		MergeBase:  compareResult.MergeBase,
+		MergeBase:  compareResult.CompareBase,
 		Type:       issues_model.PullRequestGitea,
 	}
 
@@ -1569,7 +1569,7 @@ func GetPullRequestFiles(ctx *context.APIContext) {
 		return
 	}
 
-	startCommitID := compareInfo.MergeBase
+	startCommitID := compareInfo.CompareBase
 	endCommitID := headCommitID
 
 	maxLines := setting.Git.MaxGitDiffLines