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>

templates/swagger/v1_json.tmpl

@@ -2241,6 +2241,53 @@
             "$ref": "#/responses/BranchList"
           }
         }
+      },
+      "post": {
+        "consumes": [
+          "application/json"
+        ],
+        "produces": [
+          "application/json"
+        ],
+        "tags": [
+          "repository"
+        ],
+        "summary": "Create a branch",
+        "operationId": "repoCreateBranch",
+        "parameters": [
+          {
+            "type": "string",
+            "description": "owner of the repo",
+            "name": "owner",
+            "in": "path",
+            "required": true
+          },
+          {
+            "type": "string",
+            "description": "name of the repo",
+            "name": "repo",
+            "in": "path",
+            "required": true
+          },
+          {
+            "name": "body",
+            "in": "body",
+            "schema": {
+              "$ref": "#/definitions/CreateBranchRepoOption"
+            }
+          }
+        ],
+        "responses": {
+          "201": {
+            "$ref": "#/responses/Branch"
+          },
+          "404": {
+            "description": "The old branch does not exist."
+          },
+          "409": {
+            "description": "The branch with the same name already exists."
+          }
+        }
       }
     },
     "/repos/{owner}/{repo}/branches/{branch}": {
@@ -10886,6 +10933,28 @@
       },
       "x-go-package": "code.gitea.io/gitea/modules/structs"
     },
+    "CreateBranchRepoOption": {
+      "description": "CreateBranchRepoOption options when creating a branch in a repository",
+      "type": "object",
+      "required": [
+        "new_branch_name"
+      ],
+      "properties": {
+        "new_branch_name": {
+          "description": "Name of the branch to create",
+          "type": "string",
+          "uniqueItems": true,
+          "x-go-name": "BranchName"
+        },
+        "old_branch_name": {
+          "description": "Name of the old branch to create from",
+          "type": "string",
+          "uniqueItems": true,
+          "x-go-name": "OldBranchName"
+        }
+      },
+      "x-go-package": "code.gitea.io/gitea/modules/structs"
+    },
     "CreateEmailOption": {
       "description": "CreateEmailOption options when creating email addresses",
       "type": "object",