Do not overwrite empty DefaultBranch (#22708)

Fix #21994. 
And fix #19470.

While generating new repo from a template, it does something like
"commit to git repo, re-fetch repo model from DB, and update default
branch if it's empty".


19d5b2f922/modules/repository/generate.go (L241-L253)

Unfortunately, when load repo from DB, the default branch will be set to
`setting.Repository.DefaultBranch` if it's empty:


19d5b2f922/models/repo/repo.go (L228-L233)

I believe it's a very old temporary patch but has been kept for many
years, see:
[2d2d85bb](https://github.com/go-gitea/gitea/commit/2d2d85bb#diff-1851799b06733db4df3ec74385c1e8850ee5aedee70b8b55366910d22725eea8)

I know it's a risk to delete it, may lead to potential behavioral
changes, but we cannot keep the outdated `FIXME` forever. On the other
hand, an empty `DefaultBranch` does make sense: an empty repo doesn't
have one conceptually (actually, Gitea will still set it to
`setting.Repository.DefaultBranch` to make it safer).

models/fixtures/repository.yml

   owner_name: user2
   lower_name: repo1
   name: repo1
+  default_branch: master
   num_watches: 4
   num_stars: 0
   num_forks: 0
   owner_name: user2
   lower_name: repo2
   name: repo2
+  default_branch: master
   num_watches: 0
   num_stars: 1
   num_forks: 0
   owner_name: user3
   lower_name: repo3
   name: repo3
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: user5
   lower_name: repo4
   name: repo4
+  default_branch: master
   num_watches: 0
   num_stars: 1
   num_forks: 0
   owner_name: user12
   lower_name: repo10
   name: repo10
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 1
   owner_name: user13
   lower_name: repo11
   name: repo11
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: user2
   lower_name: repo15
   name: repo15
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: user2
   lower_name: repo16
   name: repo16
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: user2
   lower_name: repo20
   name: repo20
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: user2
   lower_name: utf8
   name: utf8
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: user2
   lower_name: commits_search_test
   name: commits_search_test
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: user2
   lower_name: git_hooks_test
   name: git_hooks_test
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: limited_org
   lower_name: public_repo_on_limited_org
   name: public_repo_on_limited_org
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: limited_org
   lower_name: private_repo_on_limited_org
   name: private_repo_on_limited_org
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: privated_org
   lower_name: public_repo_on_private_org
   name: public_repo_on_private_org
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: privated_org
   lower_name: private_repo_on_private_org
   name: private_repo_on_private_org
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: user2
   lower_name: glob
   name: glob
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: user27
   lower_name: template1
   name: template1
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: org26
   lower_name: repo_external_tracker
   name: repo_external_tracker
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: org26
   lower_name: repo_external_tracker_numeric
   name: repo_external_tracker_numeric
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: org26
   lower_name: repo_external_tracker_alpha
   name: repo_external_tracker_alpha
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: user27
   lower_name: repo49
   name: repo49
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: user30
   lower_name: repo50
   name: repo50
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: user30
   lower_name: repo51
   name: repo51
+  default_branch: master
   num_watches: 0
   num_stars: 0
   num_forks: 0
   owner_name: user30
   lower_name: renderer
   name: renderer
+  default_branch: master
   is_archived: false
   is_empty: false
   is_private: false
   owner_name: user2
   lower_name: lfs
   name: lfs
+  default_branch: master
   is_empty: false
   is_archived: false
   is_private: true

models/repo/repo.go

 
 // AfterLoad is invoked from XORM after setting the values of all fields of this object.
 func (repo *Repository) AfterLoad() {
-	// FIXME: use models migration to solve all at once.
-	if len(repo.DefaultBranch) == 0 {
-		repo.DefaultBranch = setting.Repository.DefaultBranch
-	}
-
 	repo.NumOpenIssues = repo.NumIssues - repo.NumClosedIssues
 	repo.NumOpenPulls = repo.NumPulls - repo.NumClosedPulls
 	repo.NumOpenMilestones = repo.NumMilestones - repo.NumClosedMilestones