Add API Endpoint for Branch Creation (#11607)

* [FEATURE] [API] Add Endpoint for Branch Creation

Issue: https://github.com/go-gitea/gitea/issues/11376

This commit introduces an API endpoint for branch creation.

The added route is POST /repos/{owner}/{repo}/branches.
A JSON with the name of the new branch and the name of the old branch is
required as parameters.

Signed-off-by: Terence Le Huu Phuong <terence@qwasar.io>

* Put all the logic into CreateBranch and removed CreateRepoBranch

* - Added the error ErrBranchDoesNotExist in error.go
- Made the CreateNewBranch function return an errBranchDoesNotExist error
when the OldBranch does not exist
- Made the CreateBranch API function checks that the repository is not
empty and that branch exists.

* - Added a resetFixtures helper function in integration_test.go to
fine-tune test env resetting
- Added api test for CreateBranch
- Used resetFixture instead of the more general prepareTestEnv in the
repo_branch_test CreateBranch tests

* Moved the resetFixtures call inside the loop for APICreateBranch function

* Put the prepareTestEnv back in repo_branch_test

* fix import order/sort api branch test

Co-authored-by: zeripath <art27@cantab.net>

routers/api/v1/api.go

@@ -665,6 +665,7 @@ func RegisterRoutes(m *macaron.Macaron) {
 					m.Get("", repo.ListBranches)
 					m.Get("/*", context.RepoRefByType(context.RepoRefBranch), repo.GetBranch)
 					m.Delete("/*", reqRepoWriter(models.UnitTypeCode), context.RepoRefByType(context.RepoRefBranch), repo.DeleteBranch)
+					m.Post("", reqRepoWriter(models.UnitTypeCode), bind(api.CreateBranchRepoOption{}), repo.CreateBranch)
 				}, reqRepoReader(models.UnitTypeCode))
 				m.Group("/branch_protections", func() {
 					m.Get("", repo.ListBranchProtections)