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.

routers/api/v1/api.go

@@ -983,6 +983,8 @@ func Routes() *web.Route {
 			m.Post("/migrate", reqToken(), bind(api.MigrateRepoOptions{}), repo.Migrate)
 
 			m.Group("/{username}/{reponame}", func() {
+				m.Get("/compare/*", reqRepoReader(unit.TypeCode), repo.CompareDiff)
+
 				m.Combo("").Get(reqAnyRepoReader(), repo.Get).
 					Delete(reqToken(), reqOwner(), repo.Delete).
 					Patch(reqToken(), reqAdmin(), bind(api.EditRepoOption{}), repo.Edit)