Allow configuring default PR base branch (fixes #36412) (#36425)

This adds a per-repository default PR base branch and wires it through
PR entry points. It updates compare links and recently pushed branch
prompts to respect the configured base branch, and prevents auto-merge
cleanup from deleting the configured base branch on same-repo PRs.

## Behavior changes
- New PR compare links on repo home/issue list and branch list honor the
configured default PR base branch.
- The "recently pushed new branches" prompt now compares against the
configured base branch.
- Auto-merge branch cleanup skips deleting the configured base branch
(same-repo PRs only).

---------

Signed-off-by: Louis <116039387+tototomate123@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

routers/web/repo/compare.go

@@ -247,7 +247,7 @@ func ParseCompareInfo(ctx *context.Context) *git_service.CompareInfo {
 	}
 
 	// 4 get base and head refs
-	baseRefName := util.IfZero(compareReq.BaseOriRef, baseRepo.DefaultBranch)
+	baseRefName := util.IfZero(compareReq.BaseOriRef, baseRepo.GetPullRequestTargetBranch(ctx))
 	headRefName := util.IfZero(compareReq.HeadOriRef, headRepo.DefaultBranch)
 
 	baseRef := ctx.Repo.GitRepo.UnstableGuessRefByShortName(baseRefName)
@@ -276,10 +276,10 @@ func ParseCompareInfo(ctx *context.Context) *git_service.CompareInfo {
 	ctx.Data["BaseBranch"] = baseRef.ShortName() // for legacy templates
 	ctx.Data["HeadUser"] = headOwner
 	ctx.Data["HeadBranch"] = headRef.ShortName() // for legacy templates
-	ctx.Repo.PullRequest.SameRepo = isSameRepo
-
 	ctx.Data["IsPull"] = true
 
+	context.InitRepoPullRequestCtx(ctx, baseRepo, headRepo)
+
 	// The current base and head repositories and branches may not
 	// actually be the intended branches that the user wants to
 	// create a pull-request from - but also determining the head