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>

modules/structs/repo.go

@@ -160,6 +160,22 @@ type EditRepoOption struct {
 	Archived *bool `json:"archived,omitempty"`
 }
 
+// CreateBranchRepoOption options when creating a branch in a repository
+// swagger:model
+type CreateBranchRepoOption struct {
+
+	// Name of the branch to create
+	//
+	// required: true
+	// unique: true
+	BranchName string `json:"new_branch_name" binding:"Required;GitRefName;MaxSize(100)"`
+
+	// Name of the old branch to create from
+	//
+	// unique: true
+	OldBranchName string `json:"old_branch_name" binding:"GitRefName;MaxSize(100)"`
+}
+
 // TransferRepoOption options when transfer a repository's ownership
 // swagger:model
 type TransferRepoOption struct {