Fix git commit committer parsing and add some tests (#35007)

* Fix #34991
* Fix #34882

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/git/commit.go

@@ -22,9 +22,9 @@ import (
 type Commit struct {
 	Tree // FIXME: bad design, this field can be nil if the commit is from "last commit cache"
 
-	ID            ObjectID // The ID of this commit object
-	Author        *Signature
-	Committer     *Signature
+	ID            ObjectID
+	Author        *Signature // never nil
+	Committer     *Signature // never nil
 	CommitMessage string
 	Signature     *CommitSignature