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/web/repo/compare.go

@@ -35,6 +35,7 @@ import (
 	api "code.gitea.io/gitea/modules/structs"
 	"code.gitea.io/gitea/modules/typesniffer"
 	"code.gitea.io/gitea/modules/util"
+	"code.gitea.io/gitea/routers/common"
 	"code.gitea.io/gitea/services/context"
 	"code.gitea.io/gitea/services/context/upload"
 	"code.gitea.io/gitea/services/gitdiff"
@@ -185,21 +186,10 @@ func setCsvCompareContext(ctx *context.Context) {
 	}
 }
 
-// CompareInfo represents the collected results from ParseCompareInfo
-type CompareInfo struct {
-	HeadUser         *user_model.User
-	HeadRepo         *repo_model.Repository
-	HeadGitRepo      *git.Repository
-	CompareInfo      *git.CompareInfo
-	BaseBranch       string
-	HeadBranch       string
-	DirectComparison bool
-}
-
 // ParseCompareInfo parse compare info between two commit for preparing comparing references
-func ParseCompareInfo(ctx *context.Context) *CompareInfo {
+func ParseCompareInfo(ctx *context.Context) *common.CompareInfo {
 	baseRepo := ctx.Repo.Repository
-	ci := &CompareInfo{}
+	ci := &common.CompareInfo{}
 
 	fileOnly := ctx.FormBool("file-only")
 
@@ -576,7 +566,7 @@ func ParseCompareInfo(ctx *context.Context) *CompareInfo {
 // PrepareCompareDiff renders compare diff page
 func PrepareCompareDiff(
 	ctx *context.Context,
-	ci *CompareInfo,
+	ci *common.CompareInfo,
 	whitespaceBehavior git.TrustedCmdArgs,
 ) bool {
 	var (