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

tests/integration/api_packages_cargo_test.go

@@ -83,7 +83,7 @@ func testPackageCargo(t *testing.T, _ *neturl.URL) {
 		commit, err := gitRepo.GetBranchCommit(repo.DefaultBranch)
 		assert.NoError(t, err)
 
-		blob, err := commit.GetBlobByPath(path)
+		blob, err := commit.GetBlobByPath(t.Context(), gitRepo, path)
 		assert.NoError(t, err)
 
 		content, err := blob.GetBlobContent(1024)