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.

tests/integration/api_repo_compare_test.go

@@ -0,0 +1,38 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package integration
+
+import (
+	"net/http"
+	"testing"
+
+	auth_model "code.gitea.io/gitea/models/auth"
+	"code.gitea.io/gitea/models/unittest"
+	user_model "code.gitea.io/gitea/models/user"
+	api "code.gitea.io/gitea/modules/structs"
+	"code.gitea.io/gitea/tests"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestAPICompareBranches(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	user := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
+	// Login as User2.
+	session := loginUser(t, user.Name)
+	token := getTokenForLoggedInUser(t, session, auth_model.AccessTokenScopeWriteRepository)
+
+	repoName := "repo20"
+
+	req := NewRequestf(t, "GET", "/api/v1/repos/user2/%s/compare/add-csv...remove-files-b", repoName).
+		AddTokenAuth(token)
+	resp := MakeRequest(t, req, http.StatusOK)
+
+	var apiResp *api.Compare
+	DecodeJSON(t, resp, &apiResp)
+
+	assert.Equal(t, 2, apiResp.TotalCommits)
+	assert.Len(t, apiResp.Commits, 2)
+}