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/middlewares.go

@@ -9,6 +9,7 @@ import (
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/optional"
+	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/util"
 	"code.gitea.io/gitea/services/context"
 	"code.gitea.io/gitea/services/gitdiff"
@@ -80,6 +81,14 @@ func SetWhitespaceBehavior(ctx *context.Context) {
 	}
 }
 
+func GetWhitespaceBehavior(ctx *context.Context) string {
+	behavior, ok := ctx.Data["WhitespaceBehavior"].(string)
+	if !ok {
+		setting.PanicInDevOrTesting("WhitespaceBehavior is not set in context data or is not a string")
+	}
+	return behavior
+}
+
 // SetShowOutdatedComments set the show outdated comments option as context variable
 func SetShowOutdatedComments(ctx *context.Context) {
 	showOutdatedCommentsValue := ctx.FormString("show-outdated")
@@ -95,3 +104,11 @@ func SetShowOutdatedComments(ctx *context.Context) {
 	}
 	ctx.Data["ShowOutdatedComments"], _ = strconv.ParseBool(showOutdatedCommentsValue)
 }
+
+func GetShowOutdatedComments(ctx *context.Context) bool {
+	show, ok := ctx.Data["ShowOutdatedComments"].(bool)
+	if !ok {
+		setting.PanicInDevOrTesting("ShowOutdatedComments is not set in context data or is not a bool")
+	}
+	return show
+}