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>

models/error.go

@@ -995,6 +995,21 @@ func IsErrWontSign(err error) bool {
 //  |______  / |__|  (____  /___|  /\___  >___|  /
 //         \/             \/     \/     \/     \/
 
+// ErrBranchDoesNotExist represents an error that branch with such name does not exist.
+type ErrBranchDoesNotExist struct {
+	BranchName string
+}
+
+// IsErrBranchDoesNotExist checks if an error is an ErrBranchDoesNotExist.
+func IsErrBranchDoesNotExist(err error) bool {
+	_, ok := err.(ErrBranchDoesNotExist)
+	return ok
+}
+
+func (err ErrBranchDoesNotExist) Error() string {
+	return fmt.Sprintf("branch does not exist [name: %s]", err.BranchName)
+}
+
 // ErrBranchAlreadyExists represents an error that branch with such name already exists.
 type ErrBranchAlreadyExists struct {
 	BranchName string