Add API to get PR by base/head (#29242)

Closes https://github.com/go-gitea/gitea/issues/16289

Add a new API `/repos/{owner}/{repo}/pulls/{base}/{head}` to get a PR by
its base and head branch.

templates/swagger/v1_json.tmpl

@@ -10409,6 +10409,56 @@
         }
       }
     },
+    "/repos/{owner}/{repo}/pulls/{base}/{head}": {
+      "get": {
+        "produces": [
+          "application/json"
+        ],
+        "tags": [
+          "repository"
+        ],
+        "summary": "Get a pull request by base and head",
+        "operationId": "repoGetPullRequestByBaseHead",
+        "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
+          },
+          {
+            "type": "string",
+            "description": "base of the pull request to get",
+            "name": "base",
+            "in": "path",
+            "required": true
+          },
+          {
+            "type": "string",
+            "description": "head of the pull request to get",
+            "name": "head",
+            "in": "path",
+            "required": true
+          }
+        ],
+        "responses": {
+          "200": {
+            "$ref": "#/responses/PullRequest"
+          },
+          "404": {
+            "$ref": "#/responses/notFound"
+          }
+        }
+      }
+    },
     "/repos/{owner}/{repo}/pulls/{index}": {
       "get": {
         "produces": [