feat(api): implement branch/commit comparison API (#30349)

- Add new `Compare` struct to represent comparison between two commits
- Introduce new API endpoint `/compare/*` to get commit comparison
information
- Create new file `repo_compare.go` with the `Compare` struct definition
- Add new file `compare.go` in `routers/api/v1/repo` to handle
comparison logic
- Add new file `compare.go` in `routers/common` to define `CompareInfo`
struct
- Refactor `ParseCompareInfo` function to use `common.CompareInfo`
struct
- Update Swagger documentation to include the new API endpoint for
commit comparison
- Remove duplicate `CompareInfo` struct from
`routers/web/repo/compare.go`
- Adjust base path in Swagger template to be relative (`/api/v1`)

GitHub API
https://docs.github.com/en/rest/commits/commits?apiVersion=2022-11-28#compare-two-commits

---------

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
(cherry picked from commit c70e442ce4b99e2a1f1bf216afcfa1ad78d1925a)

Conflicts:
	- routers/api/v1/swagger/repo.go
	  Conflict resolved by manually adding the lines from the Gitea
	  PR.

templates/swagger/v1_json.tmpl

@@ -5210,6 +5210,51 @@
         }
       }
     },
+    "/repos/{owner}/{repo}/compare/{basehead}": {
+      "get": {
+        "produces": [
+          "application/json"
+        ],
+        "tags": [
+          "Get",
+          "commit",
+          "comparison"
+        ],
+        "summary": "Get commit comparison information",
+        "operationId": "information",
+        "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": "compare two branches or commits",
+            "name": "basehead",
+            "in": "path",
+            "required": true
+          }
+        ],
+        "responses": {
+          "200": {
+            "$ref": "#/responses/Compare"
+          },
+          "404": {
+            "$ref": "#/responses/notFound"
+          }
+        }
+      }
+    },
     "/repos/{owner}/{repo}/contents": {
       "get": {
         "produces": [
@@ -19017,6 +19062,25 @@
       },
       "x-go-package": "code.gitea.io/gitea/modules/structs"
     },
+    "Compare": {
+      "type": "object",
+      "title": "Compare represents a comparison between two commits.",
+      "properties": {
+        "commits": {
+          "type": "array",
+          "items": {
+            "$ref": "#/definitions/Commit"
+          },
+          "x-go-name": "Commits"
+        },
+        "total_commits": {
+          "type": "integer",
+          "format": "int64",
+          "x-go-name": "TotalCommits"
+        }
+      },
+      "x-go-package": "code.gitea.io/gitea/modules/structs"
+    },
     "ContentsResponse": {
       "description": "ContentsResponse contains information about a repo's entry's (dir, file, symlink, submodule) metadata and content",
       "type": "object",
@@ -25088,6 +25152,12 @@
         }
       }
     },
+    "Compare": {
+      "description": "",
+      "schema": {
+        "$ref": "#/definitions/Compare"
+      }
+    },
     "ContentsListResponse": {
       "description": "ContentsListResponse",
       "schema": {