refactor(git): clarify GetBranch behavior to make it only gets an existing branch (#38662)

`GetBranch` silently returned soft-deleted branches, contradicting
`IsBranchExist` and forcing callers to manually check `branch.IsDeleted`
everywhere.

Refactored it to `GetBranchExisting` to have a clear behavior: it only
returns the existing branch.

---------

Signed-off-by: Sudhanshu Singh <sudhanshuwriterblc@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/repo/pull.go

@@ -173,14 +173,9 @@ func (prInfo *pullRequestViewInfo) setTemplateDataMergeTarget(ctx *context.Conte
 	ctx.Data["BaseTarget"] = pull.BaseBranch
 	headBranchLink := ""
 	if pull.Flow == issues_model.PullRequestFlowGithub {
-		b, err := git_model.GetBranch(ctx, pull.HeadRepoID, pull.HeadBranch)
-		switch {
-		case err == nil:
-			if !b.IsDeleted {
-				headBranchLink = pull.GetHeadBranchLink(ctx)
-			}
-		case !git_model.IsErrBranchNotExist(err):
-			log.Error("GetBranch: %v", err)
+		headBranchExists, _ := git_model.IsBranchExist(ctx, pull.HeadRepoID, pull.HeadBranch)
+		if headBranchExists {
+			headBranchLink = pull.GetHeadBranchLink(ctx)
 		}
 	}
 	ctx.Data["HeadBranchLink"] = headBranchLink