refactor: decouple git.Repository(ctx) from git.Commit & git.Tree (#38464)

1. Storing "ctx" in a long-living object is wrong
2. Make the commit & tree cacheable (for the future performance
optimization)
3. Also fix some bad designs like `// FIXME: bad design, this field can
be nil if the commit is from "last commit cache"`

ref:
* #33893

routers/api/v1/repo/file.go

@@ -202,7 +202,7 @@ func GetRawFileOrLFS(ctx *context.APIContext) {
 }
 
 func getBlobForEntry(ctx *context.APIContext) (blob *git.Blob, entry *git.TreeEntry, lastModified *time.Time) {
-	entry, err := ctx.Repo.Commit.GetTreeEntryByPath(ctx.Repo.TreePath)
+	entry, err := ctx.Repo.Commit.GetTreeEntryByPath(ctx, ctx.Repo.GitRepo, ctx.Repo.TreePath)
 	if err != nil {
 		if git.IsErrNotExist(err) {
 			ctx.APIErrorNotFound()
@@ -224,7 +224,7 @@ func getBlobForEntry(ctx *context.APIContext) (blob *git.Blob, entry *git.TreeEn
 	}
 	when := &latestCommit.Committer.When
 
-	return entry.Blob(), entry, when
+	return entry.Blob(ctx.Repo.GitRepo), entry, when
 }
 
 // GetArchive get archive of a repository
@@ -299,7 +299,7 @@ func GetEditorconfig(ctx *context.APIContext) {
 	//   "404":
 	//     "$ref": "#/responses/notFound"
 
-	ec, _, err := ctx.Repo.GetEditorconfig(ctx.Repo.Commit)
+	ec, _, err := ctx.Repo.GetEditorconfig(ctx, ctx.Repo.Commit)
 	if err != nil {
 		ctx.APIErrorAuto(err)
 		return