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/git_misc_test.go

@@ -50,10 +50,10 @@ func TestDataAsyncDoubleRead_Issue29101(t *testing.T) {
 		commit, err := gitRepo.GetCommit(sha)
 		assert.NoError(t, err)
 
-		entry, err := commit.GetTreeEntryByPath("test.txt")
+		entry, err := commit.GetTreeEntryByPath(t.Context(), gitRepo, "test.txt")
 		assert.NoError(t, err)
 
-		b := entry.Blob()
+		b := entry.Blob(gitRepo)
 		r1, err := b.DataAsync()
 		assert.NoError(t, err)
 		defer r1.Close()