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/commits.go

@@ -235,7 +235,7 @@ func GetAllCommits(ctx *context.APIContext) {
 		}
 
 		// Query commits
-		commits, err = baseCommit.CommitsByRange(listOptions.Page, listOptions.PageSize, not, since, until)
+		commits, err = baseCommit.CommitsByRange(ctx.Repo.GitRepo, listOptions.Page, listOptions.PageSize, not, since, until)
 		if err != nil {
 			ctx.APIErrorInternal(err)
 			return